While DL was downsizing CVG and MEM, they were building up aggressively in airports such as LAX, SEA, LGA/JFK. And now, BOS and RDU. UA also further retreated in LAX during that time period.
I'd also say that CVG remains a respectable focus city for DL even now. I'm not sure why CVG can still work as a strong focus city for DL, but not CLE for UA. Also, when UA exited some routes from CLE such as BOS, DL was the first to come in.
DL at CVG is a different story. They realized they cut too much and actually added back and is still a Focus City or borderline Hub. There's been a considerable amount of frequencies added and/or mainline upgrades and some routes even brought back from the dead (like CVG-DEN.) I'm still confused as to why UA hasn't done this at CLE, or at other large spokes for PTP routes like DL does. For UA that would be FLL, MCO, SAT, AUS, BOS and CLE. UA seems to be hellbent on hub and spoke configuration(s) and not focus cities.
One big difference between CVG and CLE is that DL still has nonstop service out of CVG to some non-hub destinations outside of Florida such as AUS, BWI, CLT, ORD, DFW, DEN, BDL, IAH, MCI, LAS, EWR, PHL, RDU, STL, SFO, YYZ, and DCA, whereas UA's remaining nonstop routes out of CLE will be to hubs or Florida destinations once UA discontinues CLE-LGA and CLE-DCA nonstop service.
DL also currently doesn't have any nonstop competition on its CVG-BDL, CVG-MCI, CVG-RDU, CVG-STL, or CVG-SFO nonstop routes, whereas UA has nonstop competition on most of its remaining nonstop routes out of CLE. DL is also currently the only airline serving its BOS, DTW, SLC, and SEA hubs nonstop from CVG.
Houston is also the only remaining market that is served nonstop out of CLE on UA but not on any of its competitors, even though WN had previously stated that it was considering adding CLE-HOU nonstop service.