Why not to add a 'slow' CPU mining option to the full-node software?
Am I missing something here?
For starters it requires a hard fork to make a major change in bitcoin consensus. That means the benefit of such a change has to be significantly high for it to even be considered by the network.
No no, I think he means simply adding back the CPU mining capability to Bitcoin Core. It would definitely be super slow and unprofitable, so you would run it without pool (decentralizing hashrate) and in case large parts of the mining network go down, diff will adjust and all the full nodes together could act as sort of a fallback. The issue though is the scales we're talking about. ASICs are not just a bit faster than CPUs, they're orders of magnitude faster. So I'm not sure if even all Bitcoin nodes ran a CPU miner in the background, it would help in any type of attack.
The idea though (decentralizing not only nodes but also hashpower) is currently embodied in the
FutureBit Apollo project.