I think he does have a chance he might actually win..
He just didn't want to, Dooku doesn't seem as Dark as a Sith should be to me. More like somebody that just wants to change things who is unhappy with the Jedi Order and the republic and wants them to change. If he would have wanted to he could have confronted Sidious and unless it would turn out to be a duel of force powers Dooku would win, and even with force powers Sidious isn't all that much better then Dooku. Yes he is better but not necessarily enough to make him win that battle easily. Not to mention that Sidious his most powerful attack could be blocked with a lightsaber.
The fight however would not be worth the time for Dooku and especially not at that moment when the Republic hasn't even fallen yet, when his war wasn't finished yet. Even if Dooku would have been alive after Rots I doubt he would have killed Sidious. He seems like a loyal little fellow