>>>> Our town elected to try and preserve only the most mature trees on public ground. I don't think they realized that it would cost them more money to cut the trees down rather than treat them.
Are you sure? One has to weigh the total costs. Keep in mind that trees would need to be treated in perpetuity, rather than a one time cost. Anything living is not static. It's not like a statue and you're not saving it forever; all living trees will die. And the bigger it gets, the more it would cost to remove.
We don't even know what the long term effects of these newer chemicals are. Traditionally, pesticides used to kill borers are very toxic, and many have already been banned. Using them in public spaces could be thought of as irresponsible. A claim that would be made by any environmentalist. Certainly, it's a consideration. There could easily be lawsuits against the city for supposed exposure.
I have to say, I side with your town on this. In fact, just this year I took down the only green ash in my yard (18 inch diameter trunk) in advance of the insect infestation here. I'd much rather get a head start on planting a tree that will naturally withstand the environment, rather than me coddling it.
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I read that the U of Minnesota predicted 80% kill of the Emerald ash borer after the early January polar vortex in Minnesota. Temps went below -20F. This second stint is not quite as cold, but is more prolonged for us, and may actually do more damage to the insect.
