Would anyone even expend the effort if effectively all of the transactions are signalling for replacement (once all wallets have opt-in RBF enabled by default)?

In other words, this could be forced through at the wallet app/edge device level, but that would take longer -- rather than this change to the relay policy that the bitcoin core client implements.

I confirm no blocks, I have no power to include a transaction that other miners will not.

Today miners and pools (or cartel of miners and pools) can include any valid transaction in a block.

That doesn't prevent attacks, it just makes the weakest and most obvious attacks to be less likely to be successful. Zeroconf is simply not safe and the sooner it is put to rest (i.e., communicate that it will be financial suicide for anyone that continues to implement it) the better, in my opinion.