I was some level of Medallion, usually Royal and after 1995 Gold, for a decade or so until I switched to AA in 1999. The L fare policy seemed too extreme for me (take more credits to upgrade L, or do it space-available at the airport, but don't rule it out completely) and the replies to my correspondence on the topic consisted entirely of form letters that missed the point. I also saw a big drop-off in service from the late 1980s to the late 1990s.

I have consistently found, for my trips, that the lowest AA fare plus the cost of purchased 500-mile upgrades is less than the lowest upgradeable DL fare. "Free" upgrades? Hah!

The revenue loss to DL from my travel and that of colleagues and friends who follow my flight choices is easily in six figures. (If anyone from Delta is reading this, it would have been better for your bottom line to have written me a decent letter. There's a message there for the future.)

YMMV, of course, but I'm a happy camper now.