What incentive do miners have to respect the Replace-By-Fee flag? Couldn't a miner treat all transactions in their mempool as if they had Replace-By-Fee enabled? After all they could always claim that they hadn't seen the original transaction.
Disrespecting it would require them modifying their software, which has ongoing administrative costs.

Otherwise, there any incentive within the system to behave otherwise. Willfully not taking income maximizing transactions isn't an incentive compatible behavior.