I left Delta in the late 90s, largely in response to changes in their FF program (LUser fares, for example) after having been some level of Medallion, usually Royal (top-tier) in their earlier program, then mostly Gold (mid-tier) in SkyMiles, every year since about 1990.

What finally pushed me over the edge was, having mailed them a letter with suggestions for constructive changes to the new policies that would have still met their business objectives, I got a form-letter reply that missed or ignored my points and was full of platitudes about what "the majority of their frequent flyers want."

Maybe the majority did. I didn't.