Use as an iPod, it's got a headphone jack which is rather nifty, also still works fine as a basic phone, you can have the battery replaced quite cheaply.
And what do I do with my new phone? Just leave it there while I use the older iPhone? No. Realistically you'll use the new phone. The old iPhone is still not going to be used. So what do we do with them now? We install Linux or we just trash it?
Sell it or repurpose it, just like anything that gets replaced (fridge, blender, car, etc..)?
Just because your old device doesn't support linux doesn't mean it's a brick. (Surely you were aware before you bought the device that it doesn't support linux).