It does seem like DL has put a lot of 50 seaters on flights out of MSP that use to have a DC-9 on the route. GFK is one of those. I remember DC-9 did the early flights and the afternoon flights to MSP. In the middle was a couple Saabs or that old Mesaba Avro 146 jet. Pretty comfortable I thought.
But other larger cities only have CRJ-200 from MSP, that you would think would easily support a 700 or bigger. CVG, DAY, OKC, YWG, MKE, BNA, SDF, OMA, ICT, CLE, CMH to name a few.