A few years ago, NJSP attempted to get towns they provide general police coverage for, to start paying for it. A few towns whined about it and through their state legislators it was shot down.
I'm guessing that towns eventually will realize that NJSP won't do it forever due to manpower issues as you stated, so where ever the agreements are favorable to one another they do it. The other thing too is that the larger Depts. are accused of charging too much for the service, so that is a barrier.
In NJ everybody wants theirs so I doubt it will go very far. But on the flip side everyone complains about their taxes being too high.
In Morris County, Mount Arlington tried to share PD service with a larger Dept., such as Jefferson or Roxbury, but as usual the residents complained so the Mount Arlington force is still in place. The same thing with the Mendhams, the richer area of the county. The residents complained.
Personally, I think this should be forced on smaller towns to save money, but NJ law does not force the issue. If only are schools consolidated NJ would be a cheaper place to live. I do notice a few towns such as Mount Olive do share other services with smaller towns such as health and bldg. insp.