I am not saying that these movies should be done or that they're necessarily bad. Some of my favorite films (e.g., Terminator, Live Die Repeat) involve time travel. It's just that, in a significant sense, they have essential narrative flaws which cannot be corrected. You either have to spackle over those flaws (don't pay attention to the man behind the curtain!) or stipulate those flaws away in an ad hoc fashion (creating more flaws when you stop to think about them). A time travel film is always going to be a fractured vase.
Logic - Traveling backwards in time allows for contradictions. You go back in time and kill your grandfather. You are never born. If you're never born, you can't kill your grandfather. There is no avoiding this problem, in principle.
At most, one may write a story in which contingently/accidentally a causal loop is self-completing (e.g., you go back in time to stop something from happening, but it turns out that you made it happen by doing so). But even here there are logical problems. Instead of killing your grandpa, what if you are you're own grandpa? To simplify this, let's say you're your own parent in a self-completed time loop. That is, you go back in time, do the nasty with mom or dad (depending on your plumbing), resulting in a baby (i.e., you). Now, we have an account of where the DNA of one parent comes from (i.e., we can track it back through billions of years of mutation, adaptation, and selection), but where did your DNA come from? Even in such case that you cased it to happen (self-completing loop) there is an uncased cause that is needed to seal the loop.
Contradictions like this are impossible, so time travel is impossible, and there is no coherent way to seriously probe the implications of time travel. You can prove anything with a contradiction, so there is no serious counter-factual investigating the "What if" of time travel in hard/serious science fiction.
Stakes - Stories are interesting because they have stakes. What we care about is at risk. Resources are scarce. Things can be lost. Without stakes stories are boring. Dreamscape, A Nightmare on Elm Street, and The Matrix all have to artificially create stakes by stipulating that if you die in dreamland, you die in reality (since when?!?). Without that stipulation, however, Freddy would just be a nuisance and Neo could camp a re-spawn until he defeated Agent Smith.
With "anything goes" time travel (i.e., those stories where our protags can arbitrarily travel to any point and time for any reason), however, nothing is ever really lost. Did someone die? Go back in time and save them. One the glaring errors of Star Trek is that they occasionally forget that they can time travel. On the Picard show, for example, Picard lazily notes that "Well, Kirk traveled in time twice to do the sling-shot maneuver to undo Q's latest f***ery. In Star Trek IV Earth is under threat from a probe, so the crew just pops back in time to solve the problem. Why should any character stay dead? Why should any problem be unsolved? With time travel you can always go back and do it again.
Thus, we have to have stakes artificially created. The baddies control the time travel (Terminator) or your can time travel so many times (Live, Die, Repeat) or your time machine is on the fritz (Back to the Future). However, once you solve the problem in the film (assuming your survive), you can always "cheat" later. There is a scene, for example, in the Bill and Ted movies where our heroes are jammed up, but then they stop and think about it. One says to the other something along the lines of, "When we get out of this later, let's plant what we need right here!" and like magic what they need turns out to be waiting for them in a cupboard (or something along those lines). In this moment, this is the smartest time-travel writing, because our heroes realize that there are not really stakes.
Exhaustion/Laziness - It's been done and done and done. It's not even done well in most cases. It's just a cheap plot device/premise. If you paint yourself in too tight of a corner, you can always get out with time travel, which encourages lazy writing.
Again, I am not saying that it should never done or that it is not fun or that time travel ruins everything. However, I would suggest that if you're writing a script and you can solve a problem (i.e., move the plot forward) without time-travel, that it is better to think of a cleverer solution than a magic wand which rewrites reality.
Logic - Traveling backwards in time allows for contradictions. You go back in time and kill your grandfather. You are never born. If you're never born, you can't kill your grandfather. There is no avoiding this problem, in principle.
At most, one may write a story in which contingently/accidentally a causal loop is self-completing (e.g., you go back in time to stop something from happening, but it turns out that you made it happen by doing so). But even here there are logical problems. Instead of killing your grandpa, what if you are you're own grandpa? To simplify this, let's say you're your own parent in a self-completed time loop. That is, you go back in time, do the nasty with mom or dad (depending on your plumbing), resulting in a baby (i.e., you). Now, we have an account of where the DNA of one parent comes from (i.e., we can track it back through billions of years of mutation, adaptation, and selection), but where did your DNA come from? Even in such case that you cased it to happen (self-completing loop) there is an uncased cause that is needed to seal the loop.
Contradictions like this are impossible, so time travel is impossible, and there is no coherent way to seriously probe the implications of time travel. You can prove anything with a contradiction, so there is no serious counter-factual investigating the "What if" of time travel in hard/serious science fiction.
Stakes - Stories are interesting because they have stakes. What we care about is at risk. Resources are scarce. Things can be lost. Without stakes stories are boring. Dreamscape, A Nightmare on Elm Street, and The Matrix all have to artificially create stakes by stipulating that if you die in dreamland, you die in reality (since when?!?). Without that stipulation, however, Freddy would just be a nuisance and Neo could camp a re-spawn until he defeated Agent Smith.
With "anything goes" time travel (i.e., those stories where our protags can arbitrarily travel to any point and time for any reason), however, nothing is ever really lost. Did someone die? Go back in time and save them. One the glaring errors of Star Trek is that they occasionally forget that they can time travel. On the Picard show, for example, Picard lazily notes that "Well, Kirk traveled in time twice to do the sling-shot maneuver to undo Q's latest f***ery. In Star Trek IV Earth is under threat from a probe, so the crew just pops back in time to solve the problem. Why should any character stay dead? Why should any problem be unsolved? With time travel you can always go back and do it again.
Thus, we have to have stakes artificially created. The baddies control the time travel (Terminator) or your can time travel so many times (Live, Die, Repeat) or your time machine is on the fritz (Back to the Future). However, once you solve the problem in the film (assuming your survive), you can always "cheat" later. There is a scene, for example, in the Bill and Ted movies where our heroes are jammed up, but then they stop and think about it. One says to the other something along the lines of, "When we get out of this later, let's plant what we need right here!" and like magic what they need turns out to be waiting for them in a cupboard (or something along those lines). In this moment, this is the smartest time-travel writing, because our heroes realize that there are not really stakes.
Exhaustion/Laziness - It's been done and done and done. It's not even done well in most cases. It's just a cheap plot device/premise. If you paint yourself in too tight of a corner, you can always get out with time travel, which encourages lazy writing.
Again, I am not saying that it should never done or that it is not fun or that time travel ruins everything. However, I would suggest that if you're writing a script and you can solve a problem (i.e., move the plot forward) without time-travel, that it is better to think of a cleverer solution than a magic wand which rewrites reality.