Not quite sure how you get from "a better first class experience" to requiring K+ to upgrade. A bit illogical to me.

What she said would point to a better product, which would be a good thing. I don't think the FAs would interpret a stricter upgrade policy as a "better first class experience."

Having said that, things were much better when K+ was required. The other part of the policy then was that the upgrade could be locked in at the time of booking for Platinum members. Delta got a revenue premium and the Platinum flyer had some degree of upgrade certainty when selecting flights.

All Delta needed to do to soften that policy and compete with other carriers, without giving away the store, was to allow upgrades at the gate from any fare but not allow any from fares less than K in advance. Instead, for those who weren't around then, they tried to require an upgrade instrument from everyone for any upgrade (i.e., 800 mile segment upgrades IIRC), and when that system was obviously going to cause revolt, they caved at the last minute and went with the unlimited upgrades for every elite from any fare.

And things have been messy ever since. IMO.