Android users can already do sideloading but in fact very few do. If Apple is forced to enable it, it will not be a super easy process and no one will do it by accident. It is unlikely to cause the kind of widespread problem you describe.
That's true... people won't accidently sideload an app.
But they might purposely sideload an app if a big developer like Meta pulls Facebook, Instagram and Messenger off the official App Store and you have to go to their website and download it.
And who knows what kind of tracking stuff Meta could put in that version of the app since it doesn't go through Apple's app review.
That's what I'm worried about.
People love to complain about Apple's app review because sometimes they allow scammy apps into the store.
But no one talks about all the apps that are blocked by app review and don't get into the store.
As soon as developers are able to offer their iOS apps on their website with no one checking them for malware... it'll be open season.
We already know Facebook is mad at Apple because of all the ad-blocking stuff that Apple does.
So you can imagine how thrilled Facebook will be about letting people download their app from their website that is full of trackers... and there's nothing to stop them.