175s and CRJ900s were already being flown before C12. There was also a company called "Compass" flying 175s that Delta inherited, but you would need to go talk to a LNWA guy as to why they allowed that operation to start up.

No 70 seaters were allowed to upguage. In fact the number of 70s was frozen. No more were allowed. One 76 was allowed for every 2 50s that left, up to the agreed upon max, which still hasn't been reached. There were mileage limits placed on the aircraft, and as soon as 1 mainline pilot was furloughed, all 76 seaters had to be immediately parked until they were converted to 70 seaters. That is a huge economic stick.

FWIW, the company wanted unlimited amounts of 80+ seat RJs to be flown at DCI, but were told to pound sand on that. The overall end result of C12 scope was to actually stop the growth of DCI and the the beginning of bringing that flying back to mainline. It is only accelerating with the 717s and whatever 100 seater that are thinking about. Where DALPA fails is in WB scope, with JVs and codeshares.