They are doing no such thing. You don't have to use side loaded apps. If a company decides to only distribute from side loading, you can just not use their app. This is effectively what you are forcing on people who would prefer side loading, so it shouldn't be a problem for you.
I want a locked down phone. So does every old person in my life. I'll literally pay extra for a locked-down phone. If you want to side load, buy a phone that allows it, don't force Apple to make one.
You, and the old people in your life, can simply stick to downloading from the official store. This change doesn't suddenly force you to live your life in a way that you're uncomfortable with.
If applications suddenly drop out of the App Store (horrendously unlikely) and force you to install them via sideloading, don't do it. It's entirely your prerogative.
Plus, I imagine Apple is going to make it quite involved to actually sideload anything. I think the old people in your life (incidentally, I know a lot more tech savvy older people than younger people, but that's neither here nor there) are safe from the nefarious clutches of added freedom.
This is still an objectively worse outcome for the person who doesn't want sideloading (fwiw, I agree). There is a non-zero chance they'll lose access to apps they like / need to keep in touch with friends if they stick to their principles and avoid all sideload-only apps.
Can you explain how this will compromise your ability to lock down your phone? Not only do you still have complete control over which apps you install, those that are installed are still subject to the same sandbox.
If you want only Apple "approved" apps, well you can still do that and you lose nothing. You can't really complain about side load only apps that aren't Apple approved then, because that is exactly what you want. But keep in mind this is orthogonal to the "locked down" level of your phone.
These days? Sadly, yes. With good reason, I cannot by default trust any software from the web.
Depending on how exactly this is done, it might yet be safe: we can have multiple roots of trust, multi multiple app stores, etc.
I'm nostalgic for the bygone days when I didn't feel even slightly concerned about spyware because no bank cards were stored on my device and tech companies didn't phone home so routinely that the EU nations unified their existing legislation into GDPR in a vain attempt to try to get them to stop.
But I don't see those days returning short of a Butlerian Jihad against anything post-2002.
