Completely FOSS isn’t completely self-sustainable either in the real world - you’d need to be using something like RISC-V with coreboot and a completely open hardware stack with zero proprietary firmware blobs in the mix + not to mention running a fully self-hosted email/cloud stack. And if you’re using a mobile phone - even a dumb one or a pinephone - then you’re not fully FOSS. I’m not aware of anyone who’s fully FOSS out there, except maybe RMS?