Okay, but sideloading is worse? AFAICT the problem we're discussing was in F-Droid doing extra verification (somewhat incorrectly, apparently) of an APK before handing it to Android to install. Regardless of F-Droid, Android will check signatures on updates against the installed version. So your response to F-Droid imperfectly checking signatures as an extra verification on first install... is to skip that entirely and do zero verification on first install? That's strictly worse for your security.

Sideloading sounds like a massively worse option than using F-Droid even with this flaw. Humans are way more likely in making mistakes, and you lose a lot of safeguards in between you and the APK when you sideload. Also, you don’t get updates as fast, which is a whole problem in itself.

So, IMO we should not fall into that trap of immediately removing apps that had a security flaw and falling back to a way worse alternative (which sideloading is) instead.