User-space code can definitely have effects on the hardware.

A program that phones home often with tracking data, thus keeping a data connection open and the processor from sleeping would absolutely have an impact on battery life and longevity. This would be code that Apple would normally block at the AppStore level.

And we saw how mad people were when Apple slowed down processing speeds to extend battery life. Can you imagine the outrage if Apple suddenly said that your battery is no longer under warranty because you installed the Facebook app directly from Facebook?

Ios has a great API and sandbox for apps, and will kill apps in the background unless they explicitly ask for permission to do additional work. It has nothing to do with sideloading apps, this security is the bare minimum for even trusted code.

The ability to run arbitrary software on a computer is not required to call it bug-free. You can't safely run whatever software you want on the computers in your car, for example.

I replied by the logic that forfeiting guarantee is unreasonable since sideloaded apps can only break as much as existing apps can.

There are good reasons to disallow any third party applications on some platforms like cars, but apple allows it and they only have a quick look at applications. The real security is in their sandbox/API.

People share and distribute modified firmware dumps and load them on their automotive ECUs. It's actually quite common.