Here we go again.
The assembled "wisdom" says that anyone developing a new aircraft model should target the existing market and go head-to-head with the current vendors, yet all the well-founded examples indicate that there is a gaping hole in the 18-24-36 seat range.
The only reason there is no apparent demand for this size of aircraft is because operators have no choice other than a bunch of machines designed and built a couple of decades before the end of the last century when the technological and socio-economic landscape was very different.
Now that public funding for major road and rail projects is drying up, inter-regional air travel has every chance of recapturing the attention of today's travellers. I, for one, can't wait for a manufacturer to deliver something small, fast and efficient because there's a market I want to exploit.