If they provided a plausible way to sideload then they'd be on stronger grounds. It's the same as Stop Killing Games. Apple shouldn't carry the burden of hosting forever, but equally they can't just make something disappear forever.
Ideally what Stop Killing Games would like is game preservation, but at minimum we need honesty/transparency about product market places. I finally know what my minimum OS lifecycle is for my Pixel phone, and I can make a comfortable purchase decision based on that.
Even Steam isn't immune to this, it simply has an good track record relative compared to most other platforms.
Apple took a ~30% cut of the sale of the product. That should calculate into it's servicing of the product. To Ross Scott's points (and many others), if you have a perpetual service but a onetime/lifetime payment, the business model will eventually not net out.
Even if something stopped working, that doesn’t mean it can never work again.
Apple aren’t obliged to keep the platform backwards compatible as long as they let people try to run their software.