It’s especially bad in D&D5e, where the artificer can create any common magical item, but it has to be selected at level up and can’t be changed, and since the game is so focused on High Fantasy, all of the common magic items are completely worthless, since the interesting stuff happens at higher rarety. In the end the system makes the Artificer a reskinned magic user where everything is worse than a plain sourcerer.
“Beneath the Monolith” was also put out by the same company that produced the original setting with its bespoke system (Numenera/Cypher System). They just know which way the wind blows and strive to maximize their profits.
Multiclassing is an optional rule in D&D5e, not allowing it should not be controversial.
In a world where there’s the system The One Ring, I don’t see a point in doing any other Middle Earth system. Perfection has already been achieved (for this specific setting).
Concerning high-level play, I think having way lower HP for everybody would also fix a lot of things, since the main issue is that battles take forever due to having to whittle down ridiculous HP sponges.
In this case I’d actually not mind that much, since D&D5e and Diablo both are pretty much the same genre (High Fantasy with a lot of fireballs). Maybe slice all HP in half to make it a bit more lethal on both sides, but that’s it.
It’s way worse for stuff like “Adventures in Middle-Earth” and “Beneath the Monolith”. Like, how? Those genres aren’t remotely the same.
This is done in movies and books to give the reader/watcher a chance to learn the setting without getting hamfisted about it, stuff is naturally explained to the character because they don’t know it either. That’s not necessary in TTRPGs, since the player can always ask out of character.
Also, it’s a way to get some character into the story the reader/watcher can identify with. This also is not necessary in TTRPGs, since there the players naturally identify with their character (at least in most games, some do that differently).