Cessna’s new SkyCourier has the potential to reinvigorate the short-haul markets, the gap is definitely in the 30-50 seat group. Right-size for small cities, and would allow for some hub overflying, at a reasonable cost. Also, in the US, it can be operated at 30 seats, which puts it into another set of rules that operators like JSX, Countour, and others already take advantage of.
The problem I think all of the under 70 seat planes have is three fold.
1. Without a major decrease in operating cost it just doesn't pencil. The skycourier works great for exactly what it's designed to do but it doesn't bring any significant fuel or personnel cost improvements over current options. Folks aren't flying 1900's or 328's today, so flying skycouriers or 328eco's will only happen with a SIGNIFICANT decrease in cost. The 328 is trying to deliver this with new engines and possible 1 pilot operations. If they get that this hurdle will be over come (and granted the skycourier might be able to do the same with the Catalyst engine or 1 pilot operations albeit how feasible those are I don't know). The Embreaer Stout if offered in civy clothes will actually have a possible leg up here in that it's attempting (who knows if it'll work) via the electric cruise to substantially change the operational cost equation.
2. You have to somehow cover the capital costs both of production and for launch of aircraft. We know these won't be big volume sellers so either you have to find another market to make the assembly line spin between civil orders, or you have to dominate the small market. ATR does this for instance, they can keep the line moving with 72's to make the 42 pencil. Cessna has FedEx to help them on the skycourier, the 328 is looking for special ops orders along with not being a clean sheet so should in theory be cheaper, and the STOUT from Embraer is hoping to cover those costs through Brazilian Air Force orders.If I had to bet money I would actually bet on the Skycourier and STOUT because they have already fixed this issue to the extent they can (they might be still too expensive for the passenger market but they at least have a better chance of lowering those costs as they've locked up the market to subsidize the passenger launch and assembly costs).
3. This is I think the toughest and the reason why we still may not see significant uptake of these small planes in the US regardless of point one and two. The US market partially due to the regulation structure, and also the dominance of a few major airlines has signifant barriers to entry for a small player on such thin routes. Now you could solve some of this with changes to pilot regulations, FA regulations, or even mandating ease of interlining between airlines. The problem is you've either got to convince folks like Skywest, Delta, Alaska, whoever to add small planes to their arsenal which seems unlikely due to staffing constraints and other barriers, or a new player would need to emerge with that as their business plan. The small carrier though will be handicapped to some extent because even though I'd love to fly to Bozeman from Boise, there's a lot of folks who would instead flying through Portland or SLC if it means they get to amass miles on Delta or Alaska.
Having said all that I really hope the 328 works out for Dornier I've always wondered if they had gotten the 328TP, 328J, 428TP and 428J off the ground if they'd have done better, there was a whole generation of stretches of 30 seater aircraft in that era that besides for the Saab2000, never got off the ground.