It is already here to be honest. I know BrowserStack and other mobile testing platforms (at Facebook and Amazon) do host real devices, both Android and iPhones, in server farms like this. Meta wrote a blog post about it: https://engineering.fb.com/2016/07/13/android/the-mobile-dev...

At one of my previous workplaces, we discussed running the Z3 theorem prover on an iPhone cluster, because they run so much faster on A series processor than a desktop Intel machine.

Reminds me of imgix, who built their product on Apple libraries so ended up having racks of macs to run their service.

Modern app click farms already have walls of iPhones.

To be fair...insane performance in a tiny cool package...iPhones would make great servers if you could order them without the screen/camera etc. :-)