There is a case to be argued in that presumably developing the rest of the game while the classes aren’t even done yet should be a bit worrying in regards to future encounter balance.
Who knows how trying to implement reactions is messing with the development of the future acts behind the scenes. Because it sure seems like the original plan was not to implement them at all, until Solasta came out and demonstrated that such an idea would be rightfully taboo.
There is a case, but it's a weak one imo. Final tweaking of encounter balance should be one of the last things done by any game developer. I imagine Larian has rough placeholder encounters for Acts 2+, which will be a relatively small amount of work to adjust months from now to account for any changes to mechanics. Add or remove an enemy, tweak enemy placement or stats, use the new template for an enemy-with-reactions, etc
Your worry will only hold true if Larian never does a final pass-through on Act 2+ encounters after finalizing the mechanics, which would be idiotic of them.
And honestly, the removal of backstab Advantage and jump+disengage probably affected encounter balance way more than the addition of any new class will be. After all, is the sorcerer class really going to be make parties more powerful? We did already have the wizard class.
