There’s two business cases to be made. Is there sufficient passenger at a profitable fare structure to support route. Then are there enough of those 50-pax routes to support design/ build a profitable number of the jets. You need to produce enough planes to amortize the non-recurring development costs. My ROM at today’s cost of development is 1500 planes, minimum and the risk of not meeting that number must be fairly low. Hard to do.
The aircraft has already been developed … it's been flying for years … calculate just updating it with new PW engines and the ATMOSPHERE interior
To upgrade a jet with athmosphere interior, even with an updated glass cockpit, is fine, no real problem.
But to upgrade a jet with new type/generation engines located at the tail section; sorry this will not be so easy, this is another story...
First, these new generation engines are around 50% heavier,
Second, these new generation engines are also 50% more expensive and also the maintenance is much more expensive, minimum 50 %. So over 18 years, capital costs + maintenance cost will be something around 10 to 15 million $ more than prior generation jet engines?
Third: These engines, as alread said, are heavier, so around 500´to 600 kg per engine and they are stronger, so you have to modify the tail, let us say around additional 50 kg per engine. So let us say, the jet with new engines is around 1.2 tons heavier.
Fourth: 1.2 t more at a jet with engines under the wing is fine, as it is in balance. And if you have to move these engines under the wing a little bit to the front and the center of the engines a little more higher, it is already not any more fine, than ithis jet is called 737 MAX. But these engines of your 50 seet jet are at the tail-section, so as you have to stay in balance, you have to put the same amount of balance weight to the front. So regardint the additional 1.2 t at the back the total weight of the jet has to be incresed by 2.4 t..
5th: If you want to get rid of the total weight increase, you have to reduce the weight at the tail section; as excanple new materials for the fins, but also this will reduce the weight of new parts only by 30 % and it will never be enough, to reduce the additional weight at the tail by more than 200 or 300 kg; and it will be expensive.
6th: 50 seet jets are used for short haul flights regular. The new generation engines have no or only low/negible benefit in fuel consumption during start and landing. So the overal benefit of these new generation engines is low on jets in short haul flight duty, especially in smaller jets. There is a reason, why there are only low number of orders for A319 NEO, 737 MAX 7 and no orders for the E175 E2, even all of these of the old generation (A319 ceo, 737 700 and E175) have been sold pretty well. The new generation engines do not work/have no financial benefit on these small jets, and your postulated 50 seets jet will be tthe smallest of them....
There are more points, but I am sure, these mentioned are already enough.
Only one last and most important: The development/upgrade with new engines will be expensive (up to 1 billion), and will need several years. I do not see, that 400 to 500 jets will be sold, only tto get back the developement costs, especially as now all CRJ are even sold below production costs.