I feel compelled to point out that most editions have awarded at least some xp for non-combat stuff. 1e and earlier had xp for gps; 2e had class-based extra awards; 3e had guidelines for xps for traps and roleplaying; 4e had skill challenges; and 5e has (optional) guidelines for awarding xp for non-combat situations (e.g. traps in DotMM).
That said, milestone xp is great at what it's great at. But for it to work well, your campaign has to have a storyline, and it needs a consistent group of pcs of the same level. If either of those things aren't part of your playstyle- for instance, if you run a sandbox with no main storyline, or if you let pcs of mixed levels adventure together- then milestone xp becomes less useful. After all, if there's no storyline, how do you decide when you've reached a milestone? And if the pcs are mixed level, how do the lower level characters catch up?
So yeah, milestone xp is okay, but limited in its usefulness to an old-skool sandbox DM like me.