Speaking as someone who hasn't really played the original games before, I think having Jaheira and Minsc in the party is totally fine so long as they aren't the only firmly good characters. If they are then I'd personally feel like we were denied fresh, new characters because those two took up time and space on the roster. And I think this matters even for newcomer players because returning characters are likely going to have a distinct vibe to them that new ones won't. I think having them return in the second game is entirely appropriate since I believe it's a direct sequel so of course there'd be some continuity of companions. It's like in Dragon Age when Varric came back for the third game. And technically Anders coming back for the second game is a case of that as well. And look at Garrus and Tali from Mass Effect. For direct sequel games, that continuity helps keep a thread going. For BG3 I think the distance makes that less necessary. However I do think having the presence of past companions is something important. However I'd have preferred something like how Leliana in Inquisition was an advisor rather than a party member. I think that would have been a better use for Jaheira, and she could have had some sort of protege (not necessarily a child) that joins you instead.
On a related note, they've announced Minst and Jaheira are gonna be in the game as companions, they've even announced Minsc's voice actor, I wonder why they haven't shown up in the marketing more alongside the other companions? It's not even like monks, and the remaining phb races where Larian's kinda said they're confirmed but they're being reticent. Minsc was the centre of a trailer and they made a whole big deal about getting his voice actor. Arguably they made a bigger deal about Matt Mercer than they have about the villains' voice actors.
My personal theory is that Larian decided on including Jaheira and Minsc very late in their development process. They originally intended to have two or three completely new "good" companions in the game. But then they got sidetracked with all manner of other things they were excited about working on, and completely dropped the ball on creating those new "good" companions. So then, at a very late stage, they decided to (re)use Jaheira and Minsc as a way of quickly bringing in two "good" companions without having to do a lot of work on them. This is what I feel happened.
