Oh man, some wildly different play styles here.
No kidding, eh?
To me, if you only award XP to the players present, you're actually dis-incentivizing the players who missed a session from returning. It's a mindset of "well, sorry I got pneumonia. Now my character is behind everyone else and I'll never catch up. Might as well just bail on the group entirely."
Fair point, but still misses the main issue: xp should be a reward for what the character does, not the player. And as the character is likely to keep on truckin' with the party* even if it doesn't have a player attached for the night then it should still accrue the xp it would have coming to it.
* - unless some sort of metagame system is in place where an absent player's PC fades into the background for that session, but 99.5% of the time that's an even worse solution as it makes no sense in the story. (the other 0.5% are the cases where the party are on a one-session mission and that PC could reasonably be left behind)
Milestone XP. Party levels when it's appropriate in your campaign. Everyone stays the same level. Balancing encounters is easier, loading the campaign with level-appropriate loot is easier.
The main issue I have with this is that it completely disincentivizes characters from taking any individual risks and to some extent incentivizes individual risk aversion (as in, "you guys handle this, I'll stay back here").
Everyone staying the same level is not in any way a goal. If a character dies* early in an adventure and isn't revived until the end, and then everyone levels up, now you've arrived at the PC earning xp while dead!
* - or gets feebleminded and can't contribute, or whatever.
The reward for showing up and playing is getting to BE THERE when the barbarian decides to grapple the big bad and dive off a cliff, leading to certain death for the BBEG and half-damage for the really angry barbarian. Or being there when the heist goes wrong and suddenly everyone's scrambling trying to escape the vaults without being recognized.
No argument here; other than these things are
a reward, as in one of several, rather than the only reward. Others are getting to hang out with friends; and watching your character (and other characters) advance and develop be this by acquired treasure, or level-ups, or both.
One way to take some focus off of level-up is to massively slow down the advancement rate from what 5e suggests.
As a player, if the only reward in the game is "here's your XP" I'm absolutely not interested in that game.
Fair enough; I wouldn't be interested in DMing players for whom the only thing that matters is levelling up.