Is your body in balance when there's an equal number of cancerous cells? Is your garden in balance with an equal amount of weeds and crops?
This question pops up seemingly regularly among fandom hubs, and it never considers that balance in The Force isn't an algebra equation. Even with the Rule of Two limiting their numbers, the Sith steadily fomented corruption, fear and misery in the Galaxy building into all out war and fascist oppression.
The Jedi's failure to respond had nothing to do with their numbers, but with their rigid adherence to dogma robbing them of the tools necessary to address the threat. Dozens of Jedi had spoken up about the rising corruption and might have been effective in fighting it were the Council not obsessed with protocol, optics and precedent--just letting Anakin access the restricted texts would have made all the difference.
Sidious was uncommonly powerful, but he didn't collapse the galaxy on his own--a string of Masters before him used a philosophy centered on aggression, deception and abuse. They take power by making things worse for others meaning their presence is a net negative for the Galaxy at large.
The Sith are a cancer, and you don't leave cancer in a body if you want it to stay healthy.