I'm sure the percentage of people who can audit the code that are on this forum is closer to 99% than 0.1%. It's not a very complex app, perhaps 1000 lines of JavaScript. It's very easy to see if anything malicious is being done. The fact is none of your wallet data ever leaves the browser so there is no server component that can be compromised. Once you have encrypted your wallet nobody can access your private key without the password (not even the extension itself).
OK fair enough - 1000 lines is certainly doable. But 99%? Not a chance. I bet only 10% of people on this forum understand the first thing about .js code