On that topic, one thing I'm not completely clear on is whether the soul connection thing applies to the Sternritter alone or all Quincy? I know the SR are the ones with the deepest connection to Yhwach, the only ones in whom he carves a piece of his soul personally, but are all Quincy carrying a bit of his soul? Or is it that they are just descendants? Or maybe when he's called the 'progenitor' of all Quincy it means that using his power he devised the Quincy arts and was somehow able to teach them to normal humans who had spirit potential for whatever reason, and the descendants of those people became the Quincy existing today.
I mean, given how Auswahlen worked it gives me the impression that every Quincy has a piece of his soul and what he did was kill them off and take it back. Plus Haschwalth did say something like "his majesty is connected to all Quincy". But then the whole theme of blood could mean that they are literally his blood descendants.
Regardless, I don't think killing Yhwach would erase all Quincy power, but then it would kind of make sense if it did as well (assuming every single Quincy does actually have a piece of his soul). Since killing him properly would entail destroying his soul, it might be the case that doing so would cause all extensions of his soul to fade away as well. On the other hand it's possible that it just means the Quincy get to keep their little piece of his soul for themselves. All speculation I guess, the closest comparison would be whatever Yama did to Yhwach. He put him out of action, but the Quincy continued to exist. But then he wasn't really dead as such, or it seems his body was but his soul was restorable. Could be that he's never truly dead as long as his body exists in some form and Quincy with soul chunks are still alive. It might be that to really erase him from existence you'd have to destroy every single Quincy, but that seems unlikely cos it would involve killing Ichigo, Uryu, Karin and Yuzu, etc. More likely you just have to destroy his body.
We have to consider the existence of the Ishida bloodline as well, which makes things a bit more complicated, given what's been insinuated. Do they have a piece of Ywach's soul? Or do their powers come from a different source? What about Letz Stil? Why was Souken so keen on keeping that technique alive? A technique that depletes a Quincy of all their power? That's quite dangerous. But it's also convenient that they have a technique to restore the powers as well. Was Letz Stil meant to separate a Quincy from Ywach's influence? I'm assuming he is the 'progenitor' as they have to had gained their power from somewhere. I don't think the first group of Quincy were normal humans either. They had to have been spiritual bodies as well.
It appears as if some Quincy may be the descendents of those who Ywach first imparted his soul onto, which would make sense, seeing as how souls are apparently "inherited", as seen when Ichigo inherited "White" and his Father's Getsuga Tenshou. The first Quincy "students" may have wished for power to defend themselves against Hollows, their natural enemy, amongst other things they needed.
And if it's a Voldemort type situation and his soul keeps him from truly dying, it wouldn't just be the Quincy that needed to die, it would be the Shinigami as well as he spread his soul to them too in this fight.