And I guarantee that the Founders would have destroyed the Cardassians later anyway, even if they won the war, because they did attack their homeworld.
The Female Changeling's words to Garak in "BROKEN LINK" imply this.
"They're dead. You're dead. Cardassia is dead."
Yes. One possibility for a sequel to DS9: The Dominion regarding the whole season 6-7 war as a prelude. Being unable to resupply their forces in the Alpha was crippling. They were just seeing how much they'd be up against. And they almost won! So the Dominion has a much larger task force on its way from the Gamma Quadrant, a slower way that uses some wormholes they know about but not as direct as the Prophet's Wormhole. The task force should arrive in 15 years or so. They will finish off all of them - the leftover Cardassians who escaped, Romulans because the plot to commit genocide against the Founders was as much Romulan as Cardassian, and the Federation for attempting genocide using the disease Section 31 created.