For some people, a director who started out great but eventually went downhill can threaten their status as a great director. I've seen people argue this for M. Night Shyamalan, John Carpenter, Francis Ford Coppola, Ridley Scott, and John McTiernan (not to imply I'm one of those people though). If we can agree that there's nothing wrong with someone who wants to be widely regarded as a great director (I would assume that most directors have this ambition), then I don't see anything wrong with Tarantino's reasoning either. I'd say it's pretty respectable.
Status is about vanity. If that's what one is in the movie game for, that's their business. But I think it's a fairly empty ambition.
Now some of those guys you mentioned simply ran out of passion for the game (John Carpenter), or lost the ability to finance their projects properly (Coppola). In those cases I get them packing it in out of total apathy towards the business, or abject frustration. I also get an artist figuring they've already said everything they need to, which very well may be the case with Tarantino. He certainly has been circling around the same themes for a long time now.
But stopping out of the presumed worry that they might damage their perfect reputation? Nah, who cares about that. I'll keep my respect more for the artists who go down swinging.