The Republic was dead long, long before Sidious popped up. They had spent thousands of years slowly dying of corruption.
This is everyone’s daily reminder that if not for the involvement of the Sith and the Megacorps, the CIS would have been 100% on the right side of history.
The Republic exploited and trashed the Rim worlds for long before the rise of the Sith Grand Plan. Turned a blind eye while corporations, pirates, and slavers ran rampant. Slowly doubled down on the human-centric bend the Republic started with. Coopted the Jedi order as basically the republics church militant, corrupting the order and helping lead to its ruination. And general ran the galaxy in an ineffective, inefficient, corrupt barron lord.
In another universe, where Dooku did not fall to the darkside, and the corps were kept on a leash, the CIS could have been a legitimate force for good to tear down the broken Republic and create a better, more fair system. Or at least would have been allowed to cecede and chart their own course.