Because it doesn't seem implausible? Apple allowing side-loading is unprecedented, we can't say from experience how companies will respond to the ability to side-load.
The closest analog we have is Android, where side-loading is permitted. Google Play is much more lenient than the App Store, so there's less incentive for companies to make their apps sideload-only than on iOS, yet some companies have already done it. Alternative software stores are also a thing there.
I'm also not against side-loading. I think, as the owner of the device, I should be able to put software on it without Apple's permission. My only point here is that there are ways in which people who don't have to may be forced into side-loading software, meaning "just don't side-load if you don't want" is too simplistic.
