You'd need to hire more than "hundreds", especially since people get mad about false positives and false negatives. How long do you think it takes a reverse engineer to completely and thoroughly vet a browser extension or mobile app? A day maybe if you are doing it quickly and longer if you are doing it thoroughly.
Now do that for every app and extension. And repeat it for every single version that is ever uploaded.
Now do that for every app and extension. And repeat it for every single version that is ever uploaded.