It's a bit late for such a drastic change to BG3 given that Larian are beginning work on their next project.
That said, experience has had similar challenges in all of Larian's titles, whereby experience gain is not particularly well paced.
Personally, I don't like the idea of combat granting experience at all. I'd much prefer a system where experience is tied directly to story progression (This is actually a rather popular system for TT D&D games, as it shifts the focus from meta-gaming around experience gain and onto just playing through the story and making decisions based on the decision itself and not what will maximize experience points)
This also includes side quests too. I don't want to feel forced to do a side quest just because it offers experience. I want to be able to make a decision to partake of the quest because I (Or my character I'm roleplaying) choose to do so.
Tying experience to things like combat and side quests creates a rift in the roleplay. Where you do certain actions because you want (Or need) experience instead of because you choose to do those actions. And in certain games (Such as DOS2) you can end up doing dumb things like completing a side quest... Then murdering the quest givers for double experience...
In addition, with experience tied to story progression, there's more freedom to add as many enemies and side quests as you want, because no matter the number and no matter how you interact with them (If you avoid combat through dialogue, sneak past enemies, fail side quests etc) a character's progression is unaffected. Meanwhile, you're able to fine tune character progression, allowing you to tie major character development (Such as feats in BG3 or spell levels in DOS) to plot points integrating the character with the story itself.
