If France doesn't care about who takes control of the territory afterwards, as long as they aren't terrorist supporting Islamists then it does have the capacity to seriously damage IS. Will it lead to world peace and happiness, no. Will it lead to an awful lot of Syrians continuing to die, yes. Will it marginally improve the lives of those suffering under IS rule, I believe so. Will it degrade IS' ability to recruit and inspire terror attacks abroad in the long run, yes.
Given a lot of poor choices you need to find the best one. Doing nothing is a conscious choice that may not be the best option.