Of course, Inherent Vice could make a good movie. But I'm not excited about Paul Thomas Anderson directing that movie (which could easily be just a rumor, of course, and even if it is true it may still not happen).
First off, of course, it is true that Robert Altman would have been a good choice. On one hand, of course, he didn't go in for doing adaptations of famous novels (perhaps because his attempt to direct Ragtime didn't work out so well), but on the other hand he spent the later part of his career deciding to do things at least partly because he had never done them before.
But I'm not convinced that Paul Thomas Anderson is his appropriate replacement, even though they were friends. I watched Boogie Nights once and it barely held my attention. It's very long, and very predictable.
Magnolia, however, is another type of beast. Even longer than Boogie Nights, but weirdly watchable (and certainly not predictable). Not a "good movie," but almost painfully heartfelt and sincere, and that's a rare quality in Hollywood these days. It's clearly Anderson's attempt at Nashville (it even has two of the same actors, which I can't imagine is a coincidence), but it's very earnest and didactic, not qualities usually associated with Altman.
(I knew somebody who saw Magnolia once and became obsessed with it, saying, "It's like a Zen koan!" Which made me want to point out that I thought koans were supposed to be short. But I didn't, because I think that was a pretty perceptive observation about what the movie was intended to be and the effect it was intended to have.)
Anyway, I don't think any of this makes Anderson the right director for Inherent Vice (of course, I'm not sure what would make somebody the right person for the job, other than being a resurrected Robert Altman).
(By the way, I guess I should turn in my cineaste card, since I have to admit that I have received much more enjoyment from the movies of Paul W. S. Anderson than I have from those of Paul Thomas Anderson.)
The trickiest part of the adaptation is probably that the movie has have a somewhat Lebowski-ish vibe throughout, and then sneak in and break your heart at the end, as the book does. Which is why the Coens couldn't do it (not that there's been any idea that they would, but it's been mentioned just because of the Lebowski connection).
Well, here is one thing I do know: Robert Downey Jr. is too old for the part of Doc Sportello. Doc is nearing 30, and Downey is 45. It wouldn't work for Doc to be 45 any more than it would work for him to be 15. If he was 45, then he would be the Dude, and then it's a whole different movie (I talked about this here).