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.