You brought up a lot of stuff lol. Oh goodness, buckle up, this post will be stupidly long
First and foremost, this is a list of major companies who, at some point, have officially held the license to produce what we would properly call Lord of the Rings "wargaming miniatures" or scale models. Most of these licenses were granted directly from Middle-earth Enterprises (which used to be called Tolkien Enterprises) via the Saul Zaentz Company. The ones in bold still have active licenses:
-Heritage Miniatures (1978-1979)
-Grenadier Miniatures (1985)
-Citadal (Games Workshop) (1985)
-Mithril Miniatures (Prince August) (1987-present)-Harlequin/Black Tree Design (1990s)
-Citadel (Games Workshop) (2001-present)-----Sabretooth Games (2001-2003)
-----Forge World (2001-present)-Knight Models (∼2011-2013)
-Wizkids (NECA) (2011-present)-Fantasy Flight Games (Embracer) (2019-present)So, there is precedent for more than one company holding an LotR miniatures license at the same time. In this instance though, Embracer owns sub-companies (Asmodee, Fantasy Flight Games, Atomic Mass Games) with a proven track-record of producing tabletop miniature wargames and miniatures-based board games, so it may feel counter-intuitive for them to keep licensing the IP to Wizkids and especially GW. Mithril Miniatures is probably far safer than GW is right now lol.
Specifically on GW's license (extant since 2001):
Games Workshop's license is through New Line (now a subsidiary of Warner Bros) via Middle-earth Enterprises to produce wargaming miniatures based on the Peter Jackson films, including some book-specific characters and events as well.
Not including smaller spin-off board games (like Wizard's Duel, Quest to Mount Doom, or Battle in Balin's Tomb), Games Workshop has used this license to create
three different miniatures ranges and
four different major game systems. The miniatures we are most familiar with, 28mm true-scale, are associated with LotR/Hobbit/Middle-earth Strategy Battle Game; and The Lord of the Rings: The War of the Ring: Mass Battles in Middle-earth. These miniatures are produced through Citadel and FW.
The other two miniatures ranges were short-lived. GW's 40mm pre-painted range, which they produced through their now-defunt subsidiary, Sabretooth Games, was associated with game The Lord of the Rings: Combat Hex. In 2005, Games Workshop released a 10mm book-inspired Hobbit range which was associated with a game called Great Battles of Middle-earth: Battle of Five Armies. BoFA was based on Warmaster.
And, specifically on the Strategy Battle Game as a rules system:
GW has actually ported the SBG twice, both times to their now-OOP historical imprint, Warhammer Historical. The first time was Legends of the Old West (a game revolving around the American Old West) and the second time was Legends of the High Seas (a game revolving around pirates). SBG itself is a great set of rules, I agree, and it would be a shame to let it go extinct. It could work well as a Warhammer Fantasy skirmish game, but I imagine the backlash ("Why didn't you bring back Mordheim instead??") would be overwhelming.
As far as shifting partners, GW can't simply just approach the Tolkien Estate and say, "We would like to just make miniatures directly from the books." The Tolkien Estate does not have the ability to give out those rights because they sold them to United Artists--those same rights are now owned by Embracer Group via Middle-earth Enterprises. GW would have to approach MEE to do anything.
The Tolkien Estate reserved publishing rights (which is why the book publication license has bounced around from publisher to publisher, subsidiary to subsidiary, including HarperCollins, Houghton Mifflin, Del Rey, Penguin Randomhouse, etc etc). I believe TV rights were not specified in the original United Artists deal, which is why Amazon was able to buy them from the Tolkien Estate directly. Amazon and GW could probably broker some kind of deal, but my guess would be that they would need to use MEE (Embracer) as an intermediary. I might be completely wrong though.
I hope GW doesn't touch Rings of Power. I have been increasingly unimpressed with the marketing the more and more they release. As a generic fantasy show, it looks meh. But as something that is supposed to be grounded in the lore, setting, and canon of Middle-earth, it looks egregiously terrible.