The question isn't whether there won't ever be backdoors, but how likely that is, how long they will stay in place undetected, and what the effects on the reputation of the product/project/committer are once they are discovered.

Bringing this up as an argument is a sensible as claiming that transparency and democracy isn't any better than a dictatorship because there is still corruption.

There's a difference between a potential backdoor and a bug. While a lot of eyeballs might oversee a bug, you have to make an effort to design your backdoor to go unnoticed.