This is the real problem. Philosophers can’t prove their own existence to themselves, how can they be expected to prove anything else?
Whereas the rest of us are easily able to prove to our full satisfaction that atoms didn't exist, asbestos was safe, white flour no different than brown... etc, etc. It's not just possible for there to be too much certainty in the world, it's common - which is what that Billionaire knew thanks to courses in Philosophy, and exploited to earn his stash 'o cash.
Incidentally Buddhists think that any proof of individuality is false or tainted, too. Best lump them in there too. (Descartes had to please the church, we can't know his private opinion, just his published works with necessary nods to the church.)
His entire philosophy was flawed right from the start. But it usually is. That's why so many philosophy texts take the first chapter (or volume) refuting philosophers who came before them.
Peter Unger's book "Ignorance" (Oxford University Press, 1975) is probably the best modern defence of extreme skepticism.
Also, Unger is pretty fun to read.