Worse, it's never good to fall into the trap of "the good old days". Art is always evolving and changing, informed by its past. It's fine to love/prefer work from a certain era, but art becomes stagnant if it does not move forward. Any great director is a link in a chain. There is what came before and there is what came after, and both are important to understand someone's legacy.
And, frankly, if you rely solely on archival footage you run the risk of never hearing from people who were more marginalized at the time, but who might have powerful things to say about the impact of certain directors/films.
Most documentary filmmakers want to sit down and interview people. A great Ken Burns primary source bonanza is all well and good, but most documentarians want to be in the room with their interviewees.
Again, which documentaries are you talking about here?
First question. How has art evolved? I would say it's devolved. And I don't only mean movies, but music as well. I don't read fiction (I prefer biographies), but from other message boards and people I know who do read fiction complain about writers today and always seem to revert to people like Thomas Wolfe, Faulkner, Shakespeare.. Most of my laments of the last 40 years (I'm almost 40) is to find people who might say, "OK, 99% of it is crap, BUT, maybe you'll like this". I keep giving "new" stuff a chance, bending backwards, even elevating them because I'm surprised by anything great being made in my lifetime. Even here, I'll give an extra point (1-10) if something is decent, and I'll do the reverse for an older movie. I can be pretty harsh on a director because I expect more from them.. I have spent a lot of time trying out newer stuff, and usually I'm let down. Why do I keep doing it? Because I'd like to see something great made in a time I was alive, especially post-2000, when I became an adult and was conscious of what was happening. I would be familiar and maybe more in-tune with something I lived through. Also, I have always wanted that feeling I see in others who look forward to a new movie by a director they love, or a new album, or seeing a concert of a band in their prime, as opposed to me having to settle for half the band, or bands from the 60/70s who can't deliver the same performance because of age. The last great movie is now 12 years old (There Will Be Blood), but it isn't anything new in terms of art-form. It has a fine script (adapter from a great book and author, Upton Sinclair, despite the movie only interpreting the first 100 pages) and a great performance by Daniel Day-Lewis, but I have no problem with that. I just don't think there's been any evolution in a long time, but again, it's no problem if there's something tangible. "Buffalo '66" is more than 20 years old, and I like it more, but its also a movie that takes place in modern times, as opposed to "There Will Be Blood" which takes place between 1894-1928.
A few days ago, I was looking up documentaries online, and passed on it because the "stars" were directors I didn't care for. I remember looking at the IMDB page to see who was included, and there was no one I was interested in. On the flip side, I've seen some good ones on directors such as Luchino Visconti, Ingmar Bergman, John Cassavetes.. Even some who aren't my favorites (Fellini, Sam Peckinpah, Orson Welles, Fassbinder). Speaking of favorites, there are NONE of my very favorite, Vittorio De Sica. The one on Bresson was actually in Dutch, and out-of-print (luckily, there is the web), nothing on Capra. There's one on John Huston, no where online. Nothing on Ken Loach, Mike Leigh, Aki Kaurismaki, etc.. But there have been times I'm watching something that is say five to ten years old, and the "new" guy on the block is no longer even talked about - it was just a commercial fad.
You mentioned leaving out those who were marginalized at the time.. I'd love to see people who weren't as "big" then, but because of availability, are brought closer just by process of elimination. I never said I wasn't interested in what other actors, editors, or anyone else they worked with, including the families of these people. I just said I'd prefer a certain kind of director. Not just those they knew, or those who were competitors, but those who matched the quality, too.
The reason I'm so serious about this is because I have nothing left. I only love music, movies, and comedy. I'm not interested in anything else. The things people say they love (family, money, video games, sports) don't interest me at all. My traveling days have been over for years, so there's more of urgency and time I can dedicate to the few things I do like. I'm also a musician, writer, and so its extra important to find great stuff, wherever I can, from whatever part of the world... But what happens after 20 years (when I first started to "study" movies), is I become more self-aware of what I like, and with the limited time we all have, I'm going to go with the probabilities -- I'm not going to deprive myself of the only pleasure I get out of life.