Lots of DMs do level up the PCs whenever they feel like it and call it "milestone." That doesn't really incentivize going after particular goals, sticking to the plot, or engaging with certain thematic challenges, which was my point. It doesn't really incentivize not caring about leveling up, which I believe was your assertion. I'm all for actual milestone XP which is a concrete goal, and it's best in my view if it's transparent to the players so they can align themselves to it and make decisions accordingly. When the game goal of leveling up is aligned with the characters' fictional goals, the game tends to work better in my experience.