What regional are they going to be able to find who 1) has enough pilots to take on Allegiant flying and 2) Has costs low enough to make it economically feasible
The answer is none. They can't staff their current flying,let alone take on any more contracts. Plus the cost of 50 and 70 seaters is way higher then an Airbus. Even if the pilots fly it for free.
Allegiant has to have the lowest casm in the industry. They would lose money by codesharing.
Plus you'd only be talking about maybe 10-30 rjs. The costs for that small of a fleet would be absurd compared to your own.