The alliance all agrees that they cannot afford to wait for the Dominion to rebuild. Yet in the same episode, they acknowledge that the Founders are still dying. No one considers simply waiting for the Dominion's entire ruling society to die off in hopes that it will cripple the Dominion.
Some of these concerns were addressed in "The Die is Cast," when the Obsidian Order/Tal Shiar joint offensive made the attempt to eliminate the Founders in one shot.
1. They don't know if there won't be other Founders out in the galaxy who, for one reason or another, did not get infected. They could easily slot into the power vacuum that resulted, and the War would continue on as normal, save that the Founder(s) leading it won't conveniently be dying.
2. If it became absolutely critical, the Vorta could continue running the war without the Founders' influence (they've been shown, on occasion, to be capable of managing things by themselves), even after death, or putting them into some form of stasis to extend their life.
3. After the theoretical death of the Founders, the Jem'Hadar would likely take off into a suicidal last act of vengeance, or as the Female Changeling predicts later on in the arc, they could issue a "last stand" order to force the Alliance to fight the remaining Dominion forces in a bitter attrition, making them pay dearly to eliminate remaining resistance.
To that end, the Alliance had to take the initiative before either the Dominion recovered its fighting capability, or before circumstances progressed to the point that a bloody slog was the only way the war would end.
