If the Silmarillion were to be adapted, I think it should be through some kind of animation. It should also be adapted in parts rather than as a whole. By analogy, Biblical stories have made successful films, but nobody has been foolhardy enough to try to adapt the whole Bible into a movie.
As much as the scale of the Silmarillion presents a difficulty, so does its tone. The undercurrent of the whole cycle is that evil triumphs by default, and that good defeats it only rarely, fleetingly, and by huge sacrifice. The heroes always die, either in a brief moment of triumph or in the most abject failure, despair and humiliation. It's a fascinating work of fiction, and it feels more like a philosophical work than straightforward storytelling. Its message seems to be that the world is fallen and that we should bare all its cruelties manfully while hoping for redemption. I think JRR's Christian faith really shows through.
While some audiences may persevere with the gloomy tone, so long as it is served with sufficient visual beauty, I don't think that tone could survive the populist vandalism that appears to be mandatory in crafting a modern blockbuster.