They were very explicit that single player will be allowed - so for those who don't like playing online, you will still be able to do so, albeit with a slightly less rich experience. I'm planning to play co-op with a private group of friends
I don't think the "single player" aspect can have much of a story line if the same game can also be played as an MMO game which do not have story lines at all.
I like the immersive story driven games like half-life, fallout4, even skyrim (although didn't care for the story). I admit I do not know all the details of this new game, but it just feels like it will be sort of like playing fallout4 in survival mode. It will get boring pretty quickly if you have to spend all your time scrounging for food and water.
I'm playing Skyrim on Switch, whcih has A and B buttons reversed. It is absolutely infurating to come back to a Bethesda game on X1 and have my buttons be backwards.