Why not to add a 'slow' CPU mining option to the full-node software?

It may not offer a practical mining revenue opportunity, however it may improve the overall network mining decentralisation.
Imagine 'CCP-like' hashrate attack:
Malicious ASIC miners prepare to shadowmine.
Honest ASIC miners go offline because of the temporary profitability reasons.
Network security weakens.
Boom!

The Bitcoin network will have additional protection the form of the hashrate of all full nodes with the CPU mining option enabled.

Am I missing something here?