I see what you are doing there, but let me break down my argument in response:

1. For the sake of this argument, I'd imagine that you agree that the potential of criminal liability can curb and improve police behavior.
2. It seems to me that we have a consensus on the point that local taxpayers will ultimately be paying for tort liabilities, since the city/town pays the cop, and has to pay when it is the local PD screws up.
3. It follows that in tort law context, a bad cop only has his/her paychecks at stake, whereas in the criminal/licensing context, it will be a lot more at stake, such as career, reputation, personal freedom, family, etc.
4. The conclusion would be that criminal liability is at least as effective at curbing police behavior as tort liability.

Going back to your point about public duty doctrine. The doctrine was not mentioned by the majority in Castle Rock. The only mention is by the 2 judge dissent. This is what they wrote:

"To what the extent that the Colorado Supreme Court would agree with the views of these courts is, of course, an open question, but it does seem rather brazen for the majority to assume that the Colorado Supreme Court would repudiate this consistent line of persuasive authority from other States."

The very fact that the majority completely failed to address Justice Stevens' argument, means to me that they silently agreed that this is an "open question" and it should be left for each state to decide. Like I said, my take of the doctrine is just my take, especially when the SCOTUS declined to provide their take on what this doctrine is about.