I’d love to be a spider under the table at the Section 31 meeting when they decided to greenlight their genocidal plan.
The Vorta and Jem’Hadar are conducting a war on behalf of the Founders, with the goal of conquering the Alpha Quadrant for the Founders to rule. The Founders have minimal direct involvement in the conduct of the war; their role consists mostly of setting the goals and broad strategies for the war.
Our plan: introduce a virus into the Great Link so that the Founders get sick and die.
There are several possibilities for what might happen next.
Best case scenario: the Vorta and Jem’Hadar, their gods being dead, decide they have no more purpose in life. They retire to Florida and/or commit suicide, the Alpha Quadrant powers are stabilized, and the Gamma Quadrant becomes an anarchic free-for-all rife with opportunity and peril.
Worst case scenario: the Vorta and Jem’Hadar, their gods being dead, decide (or are so instructed by the Founders prior to their death) that they have no more purpose in life except revenge. The war of conquest becomes a war of annihilation, with the goal of exterminating all solids indigenous to the AQ.
How did S31 decide that Scenario #1 was so likely, and Scenario #2 so unlikely, that this plan makes sense?
They had developed a cure. In the event the war went really south--which it never really did--they'd have bartered the cure for peace (and, likely, a negotiated victory). I suspect that they kept it a secret in order to keep the war going so long as it was successful--because a Fed victory in the war meant a complete end of the nuisance of Cardassia.
And it worked out great.

I agree that there would have been little point beyond an irrational genocidal intent if there were no cure.