If this is how you frame the RPG experience, we see things entirely differently.Like ... let's say pokemon. In every Pokemon Game I have full control over how much power my pokemon have by deciding how much I train them. The story is very railroady in Pokemon, but I have freedom of choice over the strength of my team. If you would put Milestones onto pokemon, like you get 10 levels for every arena you beat, Pokemon would suck, because now you have no story agency and no mechanical agency anymore.
In this instance, I would prefer milestone both as GM and player. It means thought has been put into the game. The activities are not random, vanilla, thin, boring, etc...With XP your start point is player agency and you have to reign that in.
With Milestones your start point is no player agency and you have to put that back in.
I think this is a lack of foresight and/or experience. You can do sandboxes in milestone, and even adventure paths have x,y,z ways to achieve goals and interact with the game.Yeah, but Milestone is a more extreme less subtle version of that. With XP you can do x, y and z to get XP and level up. With Milestones you can only do X to level up.
XP is nudging, Milestone is forcing.