The idea behind LOTR is that Tolkien tried to create a modern "mythos". The guy drew on and expanded a lot from existing mythology and folklore (especially Norse myths) when creating the setting of LOTR/the Hobbit. Trying to keep the story of LOTR focused on the One Ring and all the other attached rings seems... wrong?
In the grand scheme of things, Sauron is a fairly minor villain of the setting, he's merely a high ranking general of Morgoth/Melkor (name depends on the published version of Return of the King where he gets namedropped once, Morgoth being the one used later and the intended one). The rings are just the instrument Sauron used to try and gain power. Other stories don't have to keep focusing on the rings, in fact, they're probably more interesting when they don't. The other book written by Tolkien, The Hobbit (the book, not the movies) understood this; the One Ring provokes mystery but ultimately is just a tool for the actual narrative which is Bilbo and the dwarves restoring the dwarven kingdom. Sauron doesn't even play a role in it!
If someone wanted to expand on something in the "mythos" of LOTR, then it'd be much better to just... tell a new story and tie it through sharing locations and maybe a few background characters instead of fawning over The One Ring constantly. That story has been told. Tell a different story, there's plenty of possible "room" to fit it in, even if you don't own the rights to the Silmarillion.
But who am I kidding, that idea would never make it past a Marketing Executive who wants to milk the original story for what it's worth by overfocusing on stuff that doesn't need to be focused on.
Well Melkor is his "true" name (his Valar name), Morgoth is a name given to him by the elves, meaning "The Enemy" in Sindarin.
So much this. And it can be done without any additional information from the works not covered by the existing movie rights - after all, most of the information about the Third Age exists pretty much only (at least in a finished form) in the Appendices to LotR.
Story of the fall of Arnor/Arthedain, including defeating the Witch King? Story of the Kin-strife in Gondor? Fighting the Corsairs of Umbar?
In fact, a story I would pay gold to see, and which can easily be tied into the films, is the one about the exploits of young Aragorn - as a Ranger in the North, then in the courts of Gondor and Rohan, and in the lands of Rhun and Harad. Even the title writes itself: "Where the Stars are Strange"...
Anything to not actually finish the main books.
I've heard it said that Martin is a pantser rather than a plotter, so he tends to let characters go where it makes sense for them to go. The problem might be that because he knows the broad strokes of the ending, being closer to it means it's harder to organically get characters where they need to be - he has to become more of a plotter, which isn't his preferred approach -- sounds exhausting. I think the last 2 seasons of the show demonstrated what happens if you navigate this transition poorly.
Nope, never seen any MBA do that.
Is it because there isn't any other IP out there to do? They are starved for writers? Or is it these few things are all that'll actually sell/work?
This doesn't apply to House of the Dragon. They've done a great job adapting and expanding upon the source material. It does not feel soulless. These are rich worlds created by talented authors who've dedicated their lives to building them out. Audiences are happy to see these worlds explored further.
It's worse with Rings of Power because they don't have all the rights, but they wanted to take advantage of the name to increase the views, create some sort of off brand world that is there but not quite, just like sports games where one company doesn't hold all the rights and you end up playing with some weird teams like White Blue and some generic player names, because you know you can't legally use a name. Can't wait for Gonbolf next season.