I’m not quite sure what your point is, but they aren’t “two different markets”.
Sure, one is a (mostly) subscription model, but at the end of the day, they’re both digital Elder Scrolls games sold by the same publisher.
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.
They’re moderately different target audiences. ESO is probably mostly a subset of the main ES target audience. But I’d say most Elder Scrolls players won’t try ESO.
Personally, I will likely buy TESVI if it holds up to expectations (which no, aren’t all that high right now, but I’m expecting a better game than Skyrim given 15 years of tech advances and all the money Bethesda has now compared to the late 00s when they started work on Skyrim). I haven’t tried ESO though, and likely won’t because I haven’t had time for MMOs in ages. That’s not to say I haven’t put hundreds of hours into Skyrim over the years - but that’s all on my own schedule. Same can be said for most of my gamer friends.
Now there’s going to be overlap obviously, but likely TES:VI will sell way more copies (I’m talking orders of magnitude) than ESO has active subscribers, and likely won’t have a huge impact on ESO subs either - not a long term one, anyway. Actually, it might have a positive long-term impact if anything, because more people will find out about the universe and will be willing to try ESO.