I may be capable of sticking to the App Store and not side-loading apps, but many of my friends, family and non-developer coworkers are not. They’ll click on random links in their mailbox, ads from apps and when browsing the web.. the end result is that they end up accidentally having their homepage, search engine and browser hijacked, they get redirected to spam websites, links on websites gets replaced with affiliate links, their sensitive information gets stolen, etc.
I’ve never had to help fix an iPhone and I don’t know anyone who’s been infected with malware (besides e.g. TikTok secretly copying clipboard and such), however for Windows and Android then I have people reach out to me on a monthly basis because they accidentally installed something that fucked up their computer/phone and they need help fixing it.
As for F-Droid, I hope you realize that their store is even more locked down than the App Store, and it’s as far as I’m aware the only secure third-party store on Android.