I think the thing that really "solved" it for me was realizing that the vast majority of the people who watch LotR will never read the book, let alone the Silmarillion. And those who DO decide to read those after seeing the movie - the movie won't destroy it for them.

"Stanley & I have agreed on our policy : Art or Cash. Either very profitable terms indeed; or absolute author’s veto on objectionable features or alterations."

I don't think it was explicitly stated there, but I think it is clear they went for "cash". Though I recall some of his letters saying that the unexpected success of LotR meant that he wished he'd retired earlier. It was clear that there was a lot of financial struggle in his life, but that's not to say he wrote for the money of course!