As for Mile stones defenders.
Backbencher that are not actualy helping out with strats or are simply holding back in the hopes of getting rewards with little risks might be in for a nasty surprise.
Mile stones are, for me, a lazy way to level and it also have the pernicious effect of encourage the 5mwd.
First of all, for me personally, yeah, it totally is a lazy thing to a large degree.
As it stands right now, when my players go into the Root, and I roll some random monsters, that is that and eventually we'll level up. As I understand the XP system, I'd have to look up their value, multiply by group modifiers, then divide that XP amongst the party members who participated.
If I decide a door is trapped, because it makes sense that the wizard would add a spell defense to it that... shoots 6 force blasts before running out of shots and its a dex save for 1d10 damage. Done, and eventually we'll level. With XP I have to assign an appropriate number to that trap, to make it worth something to the group.
If they spend the session haggling with a group of merchants because they want to buy cannons for their ship, and buy a crew of sailors, but need to sell off something to make it work. I can do all that and the players can have a lot of fun, and eventually we'll level. With Xp, I have to assign an appropriate number to those encounters, because they made progress and completed goals, so they should be awarded, so I need to decide which number they get.
It is frankly more work than I am willing to do for the average game, since our college games last 2 semesters tops, with breaks when everyone heads home, and people miss because of illness or massive projects worth 1/3 of their grade. I don't want to punish people for that, so I'd end up giving them XP anyways, and everyone at the table would agree with that policy, so everyone is earning just about the same XP anyways.
And lastly, it is incredibly strange to me to see so many people talking about how XP is useful for preventing slacker players from mooching off the group.... This is seriously a thing people are worried about? Who spends 4-6 hours sitting around with a group of people not
playing a game, just to see imaginary numbers get bigger. If you just want to sit around and watch numbers go up, there are plenty of ways to do that with the computer.
At the end of the day, with regards to that, I will default to the advice I just recently heard about cheating on ability score rolls. If it is so important to that person to "win" D&D and it has a minimal impact on the group, just let them, because obviously they need it, and the rest of the group can continue having fun anyways. No need to go out of your way to make sure they are "contributing" enough. Just play the game.