Some years ago, BA did just that. I was a silver member then and there was no way I could maintain status. I stopped flying BA. Have you noticed how many British business travelers now fly EasyJet, Monarch and Ryanair. BA has lost so much business to LCCs that it has lost money on short-haul. Their intra-Europe product has been modified.

UA currently rewards those who spend more money (ie by BUYING a seat in the first or business cabin).

If any legacy tries this, they will lose a huge portion of the elite base. Many of us (in this economy) fly a lot but are not paying last minute prices. So many of us would lose status that this would push us to the airline (s) that does not make this change.