"Whenever the DM feels like it" is usually still combat, plot, or goals. Just with less transparency about how far you're progressing. Alternatively, an untransparent combat, plot, or goal based leveling system is basically "Whenever the DM feels like it."If the group's method for character advancement is "level up whenever the DM feels like it," which is what many people call "milestone XP" (even though it's not defined that way in the rules), what is being incentivized here? Nothing, really. Except pestering the DM periodically to level up, I suppose.
