No one in his sane mind will devote "mining" for the sake of the Bitcoin network. It is a cost intensive operation and without a prospect of profit it is meaningless.
They will get the mining revenue. If you accept that some people will mine for profit, surely it makes sense that others will mine for profit
and to help secure the network?
Moreover, others are already in control of a big chunk of the network. So "mining" is not going to mitigate any risk because the final cost for the sellers are going to be higher than the expected benefit.
The more honest people mine, the harder it is for dishonest people to get a significant fraction of the network. I think (and hope) we'll return to the original vision of a large number of miners, each with a small fraction of the network; but instead of the myriad miners being
fan boys early adopters running mining rigs in their bedrooms, they will be corporations and governments. And this will happen because as those corporations and governments increasingly depend on Bitcoin, they will also look into mining and see it as a strategic need as well as a potential profit centre and good public relations.
The issue isn't that all miners would be fraudulent but rather a small % would.
The earlier post mentioned 20%. That to me seemed like a large percentage. Where-as if just 1% of miners are fraudulent, then the chances of a successful double-spend become quite low, compared to the meagre profit from ripping off the kind of merchants that would accept zero-confirmation transactions, and the cost of setting up the mining farm, and the damage to reputation when you get caught. I think we can get to a situation where it's irrational to try. I appreciate that doesn't mean no-one will, but it will be extremely rare.
I also appreciate you are saying that some merchants have such low tolerance for the risk that even "extremely rare" won't be good enough for them.