But if LoTR hadn't accomplished what it did in its three hours, I don't think GoT would have GOTTEN its dozen hours. I think people underestimate how massively speculative fiction movies (especially fantasy and comic book adaptations) have improved in the last 20 years.
If you compare Jackson's LoTR to Ralph Bakshi's 1978 adaptation, they simply don't play in the same league at all.
That, of course, will depend on the thing Amazon is doing. And on what happens with the rights now that Christopher is gone. I personally was happy to think that he was defending The Silmarillion from a godawful adaptation -- and I just don't see any way to do it well. But there will be many who disagree, and if it has to be done, a TV series format might be the least-worst way. (See https://mythgard.org/silmfilm/, which has been treating it as an intellectual exercise for several years.)
As I understand it, Tolkien was trying to create a national mythology for England, so he wouldn't have intended the setting to be completely alien, as he wanted its archetypes to resonate with his own culture.
Also the main reason the films seem like a familiar medieval fantasy setting is that Tolkien basically invented the high fantasy genre and its tropes. I think one movie reviewer even panned the Lord of the Rings as being too generic and derivative, when in fact everyone else has been copying Tolkien for decades.
This is pretty debatable c.f. endless scholarship on the connections between LOTR and (a) Tolkein's feelings about England post-WWI and (b) Tolkein's deep connections with actual existing mythology and legend. "Different world", perhaps, but "alien world" seems a bit much.
I don't know if any film could really capture that, especially to Christoper's satisfaction. But the movies are action-adventure blockbusters, and the books really aren't. It's notable that the Battle of the Hornburg was only about a page of text, but close to an hour of screen time. That kind of shift of focus recurs throughout the adaptation. Tolkien was creating an alien world; Jackson gave us a familiar Sort Of Middle Ages Or Something.
The films did manage to capture the age of the world better than anybody could possibly have imagined. Jackson hired the best artists, famed for producing images that were better than Tolkien's own, and capturing a lot of what Christopher would treasure in the book. As a series of still images, Christopher should have found a lot to admire in the films. But the dialogue doesn't achieve the same.