I just wander, where this "knowledge", whether MP part can affect quality of SP part of the game comes from? I suppose Larian must have answered such questions in past, but is someone expecting that developer of a product would talk about it in any other, then positive way?
I don't think there's much question that making a game MP rather than SP can have some impact on the SP experience. In general, it's more complicated to develop a game for MP - it can affect the choices you make, things have to be coded differently, and it introduces a whole new layer of optimisation and potential problems. So, I'd say there's no doubt that it increases the work and skills required, which, with a finite budget, must have some impact.
But, the good thing with this style of game is that it doesn't really impact the game design, because there's no tension between the SP and the MP - in a turn based party RPG, it makes little difference whether the party is controlled by several players each moving their character, or by one player making all the moves. With other types of games, concessions have to be made to make multiplayer work.
Taking a broader view, and although MP is not my thing, I think this could be a good move for them. There's been a big resurgence of DnD among the younglings, and they're also very fond of multiplayer games, spectating on Twitch, and so on. If Larian can capture that Critical Role audience, and become known for the go-to platform for DnD cRPG groups, I think they could do very well. And a big success would mean more of these RPGs, with better budgets, and so on.
With regard to watching others play, it's not my bag, but I don't find it that hard to understand. Years ago I had a girlfriend that didn't play games, but loved watching my friends and me gaming - all the shenanigans and banter. I can definitely imagine that there's lots of people who seek that enjoyment online, and the spectating feature seems like a nice idea, to me.