Fuck exchanging band members as vital as jugular veins, that's just not going to happen and we don't want it to happen. Would it be "cool" if Kim Deal started singing for the band? I don't know. Probably, for the spectacle. But it would not be Sonic Youth.
That said, if the band's over..... who's next? Who is going to step up and occupy the position SY has held as the godfathers of a genre? They definitely hold rank, based on both age and influence, and I don't see any bands around that are fit to take their place (in the music world; not in our hearts and minds, of course.... that place is sacred).
Another meaningless speculative question, but that's what this shit is for.