But you can legislate for minimum criteria, be it a/c fit or pilot experience. Refitting your PA28 with Mode C/S, Autopilot, etc etc might not be economically viable even compaired to a newer, better fitted type.
I have to pick you up on your example of an 'almost TBO' engine. If the implication is that an almost TBO engine is less safe than one with hundreds of hours left, then the engine has been incorrectly lifed!