Eh don't shit your pants yet. Torment was a "disaster". Tyranny did better and was merely a "disappointment". Before its first weekend this game already has more concurrent players than Tyranny ever had.
If it does somewhat worse than the first game then eh, so be it. Baldur's Gate 2 never got a sequel either, maybe it was meant to be.
I think this is what you're saying, but the reason BG2 didn't get an immediate sequel wasn't due to sales, it was more b/c BioWare was sick with dealing with Black Isle and Interplay by that point (and maybe Atari, too).
I'm sure both Black Isle and Iplay wanted another BG3, but I don't think BioWare wanted to do a 3rd game with them (although they did finish off the series with ToB, and did let Black Isle continue to license the Infinity Engine).
Apparently, even the negotiations for BG2 were pretty... strained, but BioWare was in a position to negotiate better terms, so they did for BG2, but I don't know if they needed Black Isle/Interplay anymore (or the D&D franchise) to go off and do RPGs on their own and make their own ip.
Also, I think BioWare wasn't happy when Black Isle did Baldur's Gate: Dark Alliance (I don't get the impression BioWare were consulted about the name or the game at all, it was a surprise to them). While BioWare didn't own "BG," they were still tied pretty heavily to the name, and they had some concerns Dark Alliance could hurt the brand. BIS did have the rights to use BG, however.
I don't think that was the cause of the split, however, it was just more evidence of the cracks appearing in Interplay and Black Isle - the cancellation of Torn and the resultant layoffs didn't help, either.