An Archdemon's soul can only be supported by blighted creatures who don't have one themselves, hence why it survives in darkspawn, because darkspawn are soulless.
So if darkspawn themselves are soulless, that means the 7 are too right? I mean they are technically the same thing, just more powerful?
That would mean when one of the 7 "dies," it's "essence" (not soul, because they don't have one) is basically like...a negative, black hole kind of thing, like the opposite of a soul.
That's how I fanwank them being able to regenerate into *any* blighted creature, Warden or no. Their lack of soul overrides the Warden's, sort of...sickens it, whereas the Archdemon's soul is corrupted, but still a soul that can't exist with theirs, causing them both to be destroyed.
That theory is pretty convoluted even for my pretty formidable record in mental gymnastics though, so I'm sure there's a simpler reason.
Also years of watching Buffy and Angel and now this...I'm starting to get pretty sick of the word "soul" xD