It started to dwindle around 2011 when DL wouldn't allow Silvers to use premium security lanes at high yield airports like LGA. Now it's nearly impossible to upgrade unless you fly off peak.
Managed to get C+ for free tomorrow on LAX-JFK. Free booze, food, and more legroom is OK by me and better than the back of the bus.
You get used to being at the bottom of the food chain after a while. No question that being a silver on UA is worse than a silver at DL.