Other than dual-booting Android on the iPhone, I fail to see any other credibly useful reason to run the Linux kernel or some Linux distro on an iDevice.
To each to their own I guess.
To each to their own I guess.
IMHO the hardware is quite capable. Meaning that it would be a very convenient headless server with its own UPS.
Similarly to custom ROMs for Android breath new life in non supported devices, I believe running Linux on an old iDevice would be simple way to get perfectly good hardware doing something again.