I actually do not mind that the full release of the game caps at 10th level. Here are my reasons why:
1) Most D&D 5e adventures are not meant to be run with adventurers past level 10. I recently played in a Curse of Strahd campaign and our party went and handled nearly every side quest available before taking the fight to Strahd, and we were I believe level 12 and completely wrecked his shit. He did not have a chance.
2) Larian Studios is trying to make it so that spells can affect the surrounding environment and tactical aspect of the game. Not having to account for spells of 6th level or higher will help tremendously with this.
3) Baldur's Gate 3 is including not only every class in the PHB, but at a minimum 2 subclasses each. Limiting themselves to only 10 levels means that they have more time to not only make sure they get them done right, but that don't need to spend time making content such magical equipment appropriate for higher levels.
4) By limiting themselves to a level 10 cap, it leaves them open to actual expansions that continue the story, instead of DLCs that merely add new backgrounds, classes, races, spells, and feats.
5) Finally, having a level 10 cap means that Larian Studios can spend less time on classes, and more time on actual plot and story.
1) While admittedly I have not played a D&D 5e campaign, the fact that you wrecked what I assume to be the final boss at level 12 only speaks to the balance, which as accomplished as Larian is, I have complete faith in them to be able to tweak the difficulty of enemies so that you are not completely overpowered by the end of the game or completely overwhelmed by the endgame enemies.
2) Not having a full compliment of spells for a player who wants to play as a spell casting class is kind of lame and would dissuade me from wanting to play as a caster because in my experience magic users for the first 5-6 levels are basically useless.
3) This was actually my problem with BG. While I loved the game, you only really got stronger through the equipment you were wearing unless you were a spell caster. Its an old-school system which has been improved upon IMO since the growth and development of cRPGs to add in power creep from both levels and equipment combined.
4) Expansions are cool since I believe they said there wouldn't be any DLCs, but I still want a full gaming experience where I reach the maximum level that 5e is designed around which is level 20 at the moment.
5) Larian Studios is masterful at crafting worlds and stories, I don't think that neglecting the leveling system will have any tangible effect on the story.
Edit. I cant figure out how the quote system works RIP me