The Pah-Wraiths are Big Baddies who do Bad Things, but what would have actually happened if Sisko had failed? I know from this question that they were trying to get back into the wormhole, but so what? Is there anything in the canon that explains what would have happened once they Pah-Wraiths escape, took the wormhole, and...?
This is precisely what happened in The Q Gambit. In an alternate universe the wormhole was dominated by the Pah Wraith rather than the Prophets. When the wormhole was discovered by the Dominion, the Wraiths allied themselves with the Founders and allowed them free passage through the wormhole, which in turn allowed them to dominate the Alpha Quadrant.
Kira suggests that the Wraith's motives are wholly malevolent and that they simply enjoy seeing the pain and subjugation of lesser races (and that giving the Dominion access to the Alpha Quadrant is the most straightforward way of accomplishing that).
There's no good reason to assume that had the Wraiths taken control of the wormhole in the Prime timeline, that this set of events would have played out any differently.