We've been hearing the same prediction from the doomsayers for close to a decade. With FCM now firmly in place there's little point in taking it away as most F seats are now being sold anyway. Even though upgrades are now rare, I'm quite sure DL realizes it's important to keep the carrot out there and that taking it away completely would cause a mass exodus of flyers. Not going to happen.
Completely going away? Doubtful. You can tease it all day long and get upgraded once or twice a year and that keeps people 'happy'. If you take it away there will be an airline or two that will stick around and still offer upgrades.
I do think the PE on the a350 with no C+ was done knowing they remove all upgrades except for certificates.