I get why adventure paths are milestone.
But philosophically, Milestone are "Destiny" and experience is "free will".
Perseus kills Medusa as the gods desired? Level up.
Odysseus escapes Polyphemus? Wander the ocean failing to get home, no levels for you.
From a more contemporary and less extreme scenario, if the shortlings are asked to carry the One Thing to destroy it in Mount Boom while the army makes a distraction, but the shortlings get scared and run away, have to escape the Thing Wraths and dodge a dragon and a giant spider before realizing how bad the world will get if the monsters win and turn back....how does Milestone handle that?
Well, they didnt progress their "goal", it was all a player-driven sideshow.
Ok, so maybe you let players declare their goal (good milestone gm, have a cookie) and their goal was "escape to with the elves"....except now they are going back after abandoning that goal.
But in all cases it's absolutely Experience.
My group is much more sandbox. They may think two disconnected plots are part of a bigger whole and go on a tangent. Or they may decide the clues point them somewhere completely different. And maybe they just reject a mission entirely.
As a player, I dislike milestone. I want a sandbox, even if there is an overarching plot. Milestone creates a constraint of not deviating off the script, even if it makes more sense, because it will "punish" the other players by delaying leveling. It is an anti-motivation for proposing a something materially different from the obvious path.
And let's be clear, a lot of players seriously enjoy leveling, deriving a ton of satisfaction from that. Screwing that up for them is like them rushing past the social encounters I like.
Could be because none of the milestone GMs have ever announced what the milestone objectives were.