I think of the Tiger Moth I flew a few times a few years back, and the old Cub I flew more recently. Then I think of my recently deceased 1970 Massey Ferguson garden tractor. It ran like a top, but coughed out chocking exhaust, as normal operation. When it's engine had a mechanical failure, I inquired about repairs - not possible, the parts were long ago discontinued. I suppose that the manufacturer could no longer sustain horribly polluting engines.
How long will it be before engine manufacturers either will not, or will not be permitted to support the very old engine technology? We're already seeing major airframe manufacturers quietly stepping back from supporting their legacy airframes, can the engines be far behind?