Back in 2009, anynone with a Nokia could have a personal website running on their own phone. Sadly this amazing piece of tech was never widely adopted. Today’s phone are far more powerful than those Nokias both in performance and battery backup and still we don’t see anyone running a server on their phone. Why?
I think this was never implemented on phones because there’s no incentive for large corporations to work on something like this.
The battery in a phone only functions as a UPS if it’s plugged in 24/7, in which case it isn’t usable as a phone.
An old phone plugged into a car or left on top of a cupboard is still a phone. It has Bluetooth capabilities, Google Assistant, GPS for use in cars, the list goes on. People use them as wall interfaces for IoT interfaces, so why can’t you just run Home Assistant on them directly?
Plus, phones provide cellular data and actual phone calls, something that’s very expensive to plug into a raspberry pi or comparable micro server.
I have a couple rooted Android 4 phones that I could use but they would be rather slow and have security vulnerabilities. Unless someone makes a custom ROM for the obscure models I have, these problems won’t go away.