Then I, the DM, pick new milestones depending on what your PC is currently trying to accomplish. You don't have to chase the goals I create for you; you can chase any goal you like, and you'll be rewarded for progress toward it. What matters is that you have a goal within the game world.
It's the same as any other XP award system: Reward the behavior you want to see from the players. "XP for kills" rewards players who seek out combat, and punishes players who avoid fights. "XP for gold" rewards players who look for opportunities to get rich, and punishes players who don't care about plunder. "XP (or levels) for milestones" rewards players who set goals and try to accomplish them, and punishes players who bumble around doing random stuff.
		
		
	 
And so a character say like a whimsical cleric of the jester god of trickery... needs a to set a goal (or goals) to advance in your games?
Understand, i am not in disagreement with you... however... part of the reason i stopped with the "chapter" or "gaol" driven Xp was to allow for players who have characters who are fun to play and not planners, not "heading towards this thing" but actually just out and about and seeing where the wind carries them. their bumbling around is a great vehicle for lots of fun and enjoyment for many games i have seen. 
It is my experience that *some* players like "backgrounded" characters, where the now and tomorrow is hinged on and drives them towards/away elements from their past. their "story" starts in media res.
it is also my experience that *some* players enjoy or prefer what i call "frontgrounded" characters where their past is mostly not all that interesting and the player hopes to see the character get into a story ahead of them. Their "story" starts now, as the campaign starts. 
Both of those can be great (or horrible) as can lots of the area between.
So, long while back, i stopped lasering in "advancement is tied to goals and objectives" and doubled down on letting the enticements for actions and choices stay and be highlighted in the game world itself. 
help show them reasons to want to stop the goblins or benefits that can come from it or even gains to be had from helping the goblins. then let them choose based on that... not "what gives me xp.
Works for me at least. my players seem to have liked it.
Can be simple too... take every second you would spend on running xp numbers and turn it into seconds thinking about and adding in signs of neat and cool stuff that will be consequences of choices.