In my mind, there are two ways to look at the movies. When I look at it as an adaptation, then I end up with all the problems you mention - and a lot more. But when I look at it only as a movie, then they belong to the most brilliant movies (in their price range) that have ever been made. I expect them to become a classic, just like the old Star Wars movies which lasted over 20 years already. What happens after that, I don't know. Maybe we live to see a remake when the Jackson movies are outdated somewhen?
The books will surely never be forgotten. Fifty years after their publication they are as alive as they have ever been (and were even before the movies). But I fear they will end up as
this someday.
Just like the Bakshi film made its way into "Tolkieniana" to some extend, the Jackson movies will. To some fans it will be more, to some less, because some people like movies more than others in general. And there will be many (shall I dare to say: lesser) fans who prefer the movies to the books. But that does no harm, I think.
A Silmarillion movie, if well done, would be even greater to me than a LotR movie, because it's a much more difficult task. The LotR has been called unfilmable, so what are we supposed to call the Silmarillion then? It would take a dedicated director, inspired writers and a studio which really believes this can work, just like the LotR took, but to a much higher degree. If one wants to film more or less the whole book, and not just Beren and Lúthien for example, one might need at least 5 ones of 3 hours or more. I doubt not that it can be done, but, as you asked, why should it be done?
I thought about giving reasons now for quite a while and can only give this weak one, that the Silmarillion is inaccessible to many LotR and Hobbit readers and this might change after a movie. All I can really say is that at least I would truly enjoy it if I could see the magic of the book on a screen, as far as it can be caught, adding to the overall magic, not replacing it. Maybe a director and a studio will be daring enough to stay closer to the book after they got to see how well Jackson and New Line did financially.
(cross-posted with Rune)