I'm thinking the regionals will continue to have a large presence as long as the primary airline business model remains hub & spoke. That's because this model requires a secondary low-cost "feeder" network, and it's more efficient to let the regionals operate this low-cost network while the majors concentrate on their "core" business.
In my mind there could be a future market for what Republic had wanted to do (build a new p2p airline). However, consumers today are very price sensitive, and the economics to start up and efficiently scale such an airline (while competing against the incumbents) will be extremely difficult. With their troubles, I doubt Republic has the resources or will to start up a new airline, and I don't see other players eager to jump in either.
The irony in Canada is when we see possible new entrants like NewLeaf -- the CSeries in theory could be the perfect aircraft for such an operation. But in reality the numbers are just not working out for Bombardier. Low oil prices, viability of used / older aircraft, the possible economic downturn in Canada, etc., are all hurting the program.
Even for Porter -- if they still want to pursue a national jet market -- they might be better served procuring larger aircraft from Airbus or Boeing now that CYTZ is no longer a factor.