But a random developer can't exfiltrate my data, which is the WAY WAY more common scenario that the previous poster was worried about.
I agree with them. The Apple App Store prevents a lot more malware than the ability to install ANY software. I like the Mac App Store as well, its a large library of software that is virtually guaranteed to be safe from infecting your computer with a virus.
Now, I know how to sideload onto my iPhone, I have homebrew on my MBP, but my brother/sister doesn't, my aunt can't, my uncle assumed his phone was facebook, and they are safer (from viruses) for it. Make installing viruses harder!
I spent decades as the de facto IT person in my family, and when iPhone and Android came around, family members started buying MacBooks, my load got lightened a LOT.
It's great that you know how to sideload, I'm not saying everyone has to know how. I'm arguing that the option should exist regardless of if you pay for a $99/year subscription fee. It should be a part of owning the hardware you paid for and purchasing the right to control it as the user.
Your security is non-existed until you've established a realistic threat model. And if it includes targeted surveillance by nation-state actors, you've got bigger OPSEC problems than just your phone.
You're using an iPhone, buddy. If NSO Group gets a chip on their shoulder, they can install a persistent backdoor on your phone by sending you a text message. Your security is nonexistent unless you disable iMessage and JS in Safari, even then Uncle Sam has Five Eyes watching your every move for any traces of "terrorist" support.
I'm not worried about the programs I install on F-Droid. I'm worried about the competitive harm being forced onto the iPhone community by protecting Apple from righteous competition and forcing their services as a de-facto monopoly. I'm worried about the security consequences of iCloud being backdoored in soverign countries, and the users who don't have honestly secure options because of sycophants with a superiority complex.