Selfhosted

The future of selfhosted services might includes phones yes, Android most likely not.

Think about it, those phones might work right now but in 10 years their Android versions will not support anything, they wont even have root certificate updates breaking SSL, the kernel will be missing support for whatever people need and whatnot. Maybe the phones won't even boot because some key will expire somewhere... let alone security vulnerabilities.

People selfhost on 10-year old hardware right now, but they do install modern Linux distros that are well supported and up to date. I believe the most likely scenario is that at some point the "security" of most of that hardware will be broken and you'll be able to run some version of AOSP for older hardware and/or a generic Linux.

But that might not ever happen, those phones are built like hell and we've another category of hardware with similar characteristics that was never repurposed for anything after a decade - routers. It's common to see older routers that are now too slow when it comes to wifi or even CPU and although they're way more open and primitive than modern smartphones when it comes to software we usually can't even repurpose them as dumb switches with alternative / open software. OpenWRT and DD-WRT might work in some case but those are exceptions and usually those models were already supported by those firmwares. For instance there are enough Thomson / Technicolor TG784n ISP provided routers to create a second moon and the effort to break their security and create a usual firmware is so much that nobody did it. It's just easier to pay 30€ for a cheap router/switch and move on.