> you are rich enough to afford something, but poor people are not, so buying something unaffordable to the poor is a bad thing to do.

For luxuries, I agree that would be a ridiculous point to make (a poor person can't afford a new BMW, but we as a society think that's ok). But stratification among basic services that everyone should have is the opposite of equitable. This is in part why American health care is so awful if you're poor, and it's not a situation we want to duplicate with policing.

> then maybe the local government could hire on-demand security forces for when their police capacity is low (and hold them accountable and to government standards)

Not exactly the same thing, but we've done that with military contractors, and that has not worked out particularly well for accountability.