Why not keep and use them? In fact, I am working with Greg on a demo that runs the new TinyCLR on some gadgeteer hardware.
I've been using Raspberry Pi's for my projects (wideband radio receiver, in-car media player, ADS-B plane plotter, multi room audio players, ...), all running on Linux. I do have a Pi running Windows IoT, but that's only to control some stuff through (usb to) serial (if I'm able to get it working).
Even though TinyCLR OS sounds very interesting, and even if my old Gadgeteer mainboards get an update (probably not), they would still be collecting dust.
So back on topic, if anyone is interested, or knows someone who would be interested, please let me know. Maybe it can be put to good use to get kids interested in programming / electronics for example. Otherwise it'll probably end up in storage until it's totally useless...