Bethesda announced The Elder Scrolls VI five years ago at E3 2018, but the new fantasy RPG is still “five-plus years away,” according to Phil Spencer..
In the Venn diagram of people who will pay for ESO and people who will pay for ES6, I'm sure there's a ton of overlap, but there's probably some ESO players that aren't necessarily huge fans of single player games, and I know for a fact a huge portion of people who played ES5 and will play ES6 will never play ESO. They are not the exact same group of consumers.
Prior to ESO, though, Elder Scrolls was a franchise entirely marketed at people who wanted single player RPG experiences.
Even if it’s still Elder Scrolls content- a good portion of that original market is not going to have interest in a multiplayer experience. Or a subscription experience. Or a”live narrative” experience with gated content windows.
It’s a very different experience at its core, so while there may be an overlap between the two markets in the Venn Diagram, it’s still a very different market segment than a pure single player outing.
Also, the teams behind ESO and the mainline titles are not the same. The main team that made Skyrim, Oblivion and the others is focused on Starfield now, and probably for the next three-ish years with the post-release content.