Plus, if TESO is even a thing, it's because Morrowind, Oblivion and Skyrim paved the way by creating an interesting lore and gathering an audience. TES6 adds up for TESO, rather than resting from it.

ESO's story arcs, despite being within an MMORPG, can be played single player if one is feeling particularly antisocial. There's a ton of story quests since the game has been out for a decade now that you could probably fit the entirety (content hours wise) of the Morrowind/Oblivion/Skyrim quest lines into it. Probably why the game is like 100 GB lol.

Of course, as an MMO, the storyline is constrained a bit (your choices functionally don't really matter too much) since the game world can't change drastically, so you won't have an Imperial/Stormcloak type showdown that forever altered the landscape.

Still, ESO scratches the Morrowind itch, especially their latest Necrom expansion.

There's also Tamriel Rebuilt (Morrowind mod) that also has Necrom, but I haven't had a chance to check what they've done recently. (Last time I installed it, Firewatch was the farthest east they've gone but that was a long time ago).

In any case I'd imagine TES 6 would only serve to strengthen TES online and the entire TES franchise, so I don't think that's any reason for them not to make it.