From an IGN Unfiltered interview:
"The game, [as it is] right now, wouldn't run on the... last-generation base consoles. At one point we had to stop adding new animations to the game for six months because it would just run out of memory on those devices if we had. But they're a core part of our community, so we're going to support them." - Matt Firor
https://www.ign.com/articles/eso2-zenimax-online-studio-head-talks-possibility-of-elder-scrolls-online-sequel-ign-unfiltered
It’s something console owners have known in their hearts for quite some time that a number of improvements just aren’t possible due to last gen base consoles having requirements to be supported. Microsoft and Sony have no games that can only be played on PS4 Pro or X1X.
It’s nice to have confirmation that consoles presented a serious wall and that a ton of work behind the scenes was to ensure that we could all still play the game.
- Upon further reflection, Firor said players have effectively already gotten several sequels to ESO. "In fact, in many ways, Orsinium, which is the DLC we did back in 2015... that really was Version 2, and then Morrowind was Version 3... so every year, we're doing a new chapter update. But those chapters aren't just new content, there's also a lot of tech, behind the scenes, which keeps it running and makes it relevant." -