They can't have failed to notice by now that it will be a real sticking point for a lot people. This is already one of the longest threads in this feedback section, and its been brought up in many others.
I'm not terribly inspired by the idea that if the devs don't do it, then motivated modders will be able to solve everything. This doesn't strike me as a game that will be particularly easy to mod. NWN was basically a modders dream, but lack of party control, with henchmen instead of a full party kinda sank it, despite all the other innovative things that were done. It was a perennial disappointment there even with like 3 expansions and a sequel, that we never got a party control system there. And that one was by the same developer. Larian is so much closer, much much closer to the BG vibe than NWN ever was, but they are kinda shooting themselves in the foot by hamstringing us with a party limited to 4.
4 makes it feel like Valkyrie needs food!
6 is what we want lol
Divinity has a simular mod available and the games are largely based on the same coding. You can expect the same level of mod support for BG III as they really embrace their mods, even adding a large number of them in free updates to the base game.
Again, I am not saying I wouldn't prefer them to do it themselves, but at least you can be somewhat assured you'll have the ability either way.