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Android kills stuff when it feels like it wants more room to cache apps in ram. The background stuff like the media indexer takes CPU time and generates heat (these are pretty easy to overheat when used like this.) I think some of the older android userspace even has flaws exploitable over the network. etc.
I don't know about you, but my old phones struggle with room tempurature air, I'd hate to see how they do in an attic. That with the battery (most can't run without one because of spikes in the current draw) is a bit of a fire hazard.
A lot of them can be used just fine as a server with a fairly open OS. It's usually the GPU/camera/radios etc. that need special stuff, but things like USB (for networking) and flash work fine.
It's not even that hard to set up, go grab the kernel (some of qualcomm's kernels have these super crappy buildscripts that need python 2.x), build it, throw in busybox and dropbear, fastboot the image and you're good to go.
If you plug them into a Linux machine and turn on tethering they will show up as the ethernet interface usbx or so. You can then use dhclient to get an address (although you'll want to make sure it doesn't change your default gateway) or assign a reasonable address yourself.
Terminal IDE (on old phones) or termux on new ones should have enough packages to do whatever you want.
You can also grab any rootfs with the right arch and chroot into it if you have root. You can even static link busybox/dropbear and have most of the things you would use on a small server.