I think you'd have to go back to the 40s-early 50s to find NW at DET. I have an AA timetable for 1946 that has AA flying to DET, so I suspect (but can't prove) that DET was the airport in use at the time. At some point airlines deserted DET for Willow Run (YIP -- great for U of M/EMU students, but a bit of a hike for anyone else), then they gradually moved into DTW in the late 50s-mid 60s (AA was first). Would have made for some interesting interlining in the years when some were at YIP and others at DTW.

WN flew from DET for a short while in the 80s, but the short runway meant they had to short-fuel the 737s. They tried to get the runway lengthened to allow flights to PHX, and when that didn't happen (would have meant moving at least one cemetary) WN dropped DET.

Now, I can think of several airports that were mainline (although served by Convair props in most cases, maybe with the occasional DC( in some cases) in the aftermath of the RC merger that have been downgraded to Airlink-only status. APN, PLN, MQT, CWA, ATW, just off the top of my head. ABE and (IIRC) AVP also had DC9s but are now Airlink-only.