re: Nice idea - but sadly unworkable.
Two things. First, a ground speed coupled approach is perfectly acceptable and adds a hightened level of safety. In fact, it's the way the Airbus A320 is designed to be flown during an approach. And, there is NO wear and tear on the engines.
Second, during cruise flight - at the same altitudes - ground speed does not become an issue for separation purposes. IAS can be used effectively, and in many cases is used effectively for separation. ie: Trans Ocean, Non-radar environments.