That is objectively untrue. Every system that has allowed sideloading of apps has been compromised by that very feature.
I know the risks, I want to do it anyway. I don't want a nanny over my head.
For that reason, societies tightly regulate car ownership and driving. More so than phones: I don’t need a license to use a phone, nor do I have to register my ownership or have it regularly inspected.
But technology these days has this same characteristic: Others bear the costs of your decision. Every device connected to the internet is a DDOS vector.
I don’t want you deciding whether to keep your device up to date with the latest security patches, because if you (and a few million others) don’t, GitHub is down for me.
Anyway, if you don't want to deal with how an iPhone works, how about buying a different phone.