The reason BG1 had a level 10 cap was because D&D at the time was designed around maxing out at level 10. D&D has evolved since that time to be balanced around maxing out at 20 levels, and therefore BG3 should match that change. On top of that as I said before, if you only get to level 10 out of the 20 possible levels, then mages will be basically useless until level 5-6, which I assume would be halfway through the game.