A 5e crpg would be incredibly hard to balance at high levels. It's well known that the 5e CR system breaks down at levels greater than ~10 (or even 7 imo). Casters get incredibly powerful spells that can decimate groups of enemies and instantly end combats. Additionally, saving throw bonuses don't scale well with level. E.g., a fighter's Int and Wis saves will be +0 to +2 at levels 12+ against enemy spellcaster DCs of 18-22, meaning they'll almost always fail. This turns high level 5e into an extreme game of rocket tag.
Additionally, the party balance would become wack. While casters grow exponentially in power, martials notably do not and begin to contribute much less than casters to combat, exploration, and social encounters. This feature would not be great for a multiplayer-focused game such as BG3.
A live DM can fine tune encounters to match a specific party composition and tactical competency to keep things roughly balanced, but this doesn't really work in a video game. High level encounters would either completely stomp parties or be easy to the point of trivialness.
Larian hasn't historically done DLC for their games, so I doubt they're doing it here. Idk if they've stated an official reason for why they specifically chose 10 at first and then 12, but imo it's a good balance point between character growth in power and preserving any semblance of balance.
Bg1 lv7
Bg1 expansion lv16
Bg2 lv20
Bg2 tob lvl40
