As a minor point of no real consequence the Cessna T-41 Mescalero has been in service in the US airforce and army from 1964 until today, and I've been told the Diamond DA20 Katana is in
indirect service via a through a civilian contract that screens prospective pilots.
Weird nitpick, I know.
There's also a slew of drones that may or may not use efficient small piston engines or rotary varients which may or may not count as piston.
On the data aquisition side I'm willing to bet there's still a place in the US military for low, slow, ground hugging piston engine craft that run radiometrics or EM mapping.
