On a long enough timeline I think "fraudulent" miners are all but an inevitability. I could see an entity with say 20% of the network willing to accept out of band double spends for a hefty fee (either flat rate say $10 per tx, or a % of the tx amount).
Do you not think large retailers might mitigate that risk by becoming miners themselves? If Bitcoin succeeds, I would expect many entities to be willing to devote some resources to mining, to make it harder for others to control a sizeable fraction of the network. With that goal, they don't need to make a profit, although any mining income will help defray the costs.