Do any games actually do this? I've never played a game that staggered endgame content unless said endgame content just wasn't finished yet.

Regardless, I don't see the point. If you want to enjoy the new stuff before getting down to brass tacks, then do that. Alexander will still be there when you've had your fill of flying around and trying out new jobs etc. This isn't some super competitive PvP game, so there is no real 'even ground' to take into consideration. Almost everyone serious about clearing content will preorder and have early access too, so I don't think there's a reason to block Alexander during early access either really.

Guess we'll see!