Open world content makes the game feel alive, it makes it feel like a world, it is what make a good mmo in my opinion. The ability to interact with the world around you, not just your closed circle of friends. There is something about clicking the find duty or click a dungeon => warp there that just takes away so much from what makes a mmo special.

Instances close off the world and turn things into a lobby server style of gameplay. This should be limited not a focus ever. It is a path the genre has been moving which has been eating away at what makes a mmo a mmo.

Should there be instances yes, should the game be focused on them... no. There needs to be in my opinion a 50/50 mix of open world content and instanced content. The open world needs meaning as well, not just filler content.