I'm probably going to get murdered for playing Devil's Advocate, but...
Is it really the "I'm feel like I'm losing XP" that's bothering you? Or is it that you feel there is no reward of *any* kind and decades of computer games have ingrained into you a feeling that you _must_ be rewarded for every little thing you do?
If they add in "social XP", the murder hobos will just say "oh yay! I can double-dip" and they'll do the peaceful thing to get that social XP and then they'll turn around and murder the NPC anyway to get the kill XP. And then they'll post a video of being level 10 in the middle of Act 1 and everyone playing "normally" will feel bad that they're still just level 3 and wonder what they're doing wrong.
One "trap" that I realized I was falling into is the "fear of failure" trap. There are quite a few high-DC skill checks and I felt really bad when I missed them. But after playing a while, I adjusted my attitude to "well, if I fail, one door may close, but another may open" and to just go with it. Larian has done a pretty good job of not punishing you too badly for failure. (And there have been a few cases where a successful check was actually worse than failing!) So if they also don't unduly reward you for "success", I think it balances out. Besides, there is an (intangible) reward for social play -- the NPCs are still there to talk to and interact with! There's been a bunch of situations where I saw a group and just assumed they would be hostile so I attacked from stealth, only to have a short cut-scene start and then go straight to a "oh, that's how you want to play it? Attack!" option. It might still have ended in combat, but now I've got FOMO about "what if they had something important to say?"
I would hope that Larian has balanced the game assuming that the player will fight X% of the creatures in the game and that they've balanced their levelling up around that. There are fights you can't avoid (brain doggies, gnolls). Now, if Larian pegs that % too high (e.g. they assume you'll kill 50% of the creatures and yet you manage to kill only 20% and talk your way out of the rest...) then yeah, there's a problem.
But again, I would hope that this is part of why they're doing EA. How fast are people levelling? Is it too quick, is it too slow? Are people finishing EA with roughly the same XP as everyone else? If the average "good" playthrough ends with 3500XP but your average "evil" player ends with 2500 or 4500, they'll want to fix that.
One final example: Lots of games (original BG, iirc up to Pathfinder: Kingmaker) reward you (with XP) for successfully disarming traps. But not BG3! At first, I was disappointed. But then I thought about it and the reward is there -- not having the trap blow you to pieces. Sure, they could also reward you with XP because it's more tangible and obvious than "not getting blown up", but it would also be redundant. If they give out too much XP, people will just power through the levels and then they'll complain "I'm only on Act 2 of 3, but I've already hit the level cap... WTH?".
So yeah... compared to lots of other games (Fallout, etc.), not getting XP for taking the social route seems like a dumb implementation. But it's also (I think), partly expectations that people have been trained into having and maybe need to let go of (like my expecting XP for disarming traps).
I'm not going to complain if they do add it in. But I also believe that it's not the end of the world if they don't. EA is likely going to be a long time (my guess... 6 months to a year or more before final release) and so they'll have plenty of time to tweak and balance and make sure that everyone is able to level at a decent pace.
