Pre-requisites for entering end-game areas are fine, imo, as long as they are solo or relatively easy to do with pickup groups, and don't require several weeks to finish.

COP was a pain in the ass to do with pickup groups for a very long time, and really only static groups got through with it. It was a poor design choice. At the same time, totally open, no-requirement end-game content is bland, boring, and doesn't give any sense of satisfaction. I'm looking at you, WOW.

Also that said, there's really no reason not to instance end-game raids. If the argument is "well in Sea you could go to X, Y, or Z", then I can simply give Salvage as an example, or WOW's Naxxramas raid as examples of non-linear, but instanced raids.

Open area and instanced does not automatically imply linear content, that's a different design choice that should not happen and I hope the community will be up in arms about it if it does. I know I will.