I'm not telling anyone they have to or should do this, only to tell them what they can do. You don't need any special software to copy files. If you really, really want to, you can periodically plug your phone in and copy photos off over MTP, to a computer that does have something like SyncThing installed. Is it convenient, or fast, or sleek? No. But you have that option. If you don't like being forced into choices like this you are free to buy a different phone.
Though, I just want to be clear: there really is a concept of local files on iPhone, both iPhoneOS and iPadOS have a file browser which is not just cloud-synced, and there's even an unofficial SyncThing client (though they locked photos syncing behind an IAP, of course.) I don't currently use iPhone as a daily driver but I can grab an iPhoneOS device right now and save an image from any website, no problem. It's the same thing I'd do anywhere else. There is some complexity around photos versus files that's bound to confuse people, but it's really beside the point, nothing is stopping you from saving/copying/backing up files on iOS. Of course, Apple will absolutely not let you get a first-class experience with anything other than iCloud. Third party apps don't get similar capabilities. A lot of time I spent using iPhone was actually spent sitting in apps like Google Photos trying to let them actually get caught up with their syncing because Apple are dickheads. Whatever.
But I'm not going to try to convince someone that it's worth switching to Android, manually syncing files, paying for an IAP, etc. to locally back up their files. In my opinion, the value of agency can only be learned, not taught. If anyone wants help getting some of it back, I'm happy to try to help within the bounds of what they want. If they don't, it's not my place to tell them how to live. (I'm definitely upset at how badly people are being exploited, but that's a job regulators are slacking on. It's not people's job to not be exploited.)
What I'd really like are phones that don't suck, but we're done making those. You get a heavily locked-down glass slab with a huge camera bump, no expansion options, no headphone jack, and glass that breaks when you drop it 3 feet. Frankly, I'm constantly looking for ways to simply ditch the phone entirely at this point.