Yup, they did milk runs, multiple small towns in loop from the hub.

Most small town muni airports got runways long enough for jets back in the 50's and 60's. Concrete was cheap back then.

I think you can count on both (or maybe one) hands the US airports which currently have commercial service and are severely limited by performance for NB's.

The real issue is frequency. Given travel habits, one or two round-robin services was plenty back in the golden age. But today people are busy, and travel casually, so they like frequency options. More than anything else that's the RJ sweet spot.