I have to agree, cpu mining is not dead by any objective measure, btc included, though you will never ROI if doing it on a pool. But who knows, with btc you may find a block solo mining and of-course with alt coins you never know where the price is headed.
No, it is dead. 100% dead and that is the right answer here.
The question there was "How many of these (RasPI0) would you need to mine BTC, it should be worth it because they are so cheap." and that is just plain not the case. If you want something small and cheap to mine BTC, its going to be a Sidehack stick. Maybe something like a pod miner.
TLDR: RasPI0, definitively not for mining BTC. Acting as a controller, yes.