There are unsubstantiated posts on various aviation news and/or analysis websites that Boeing and Airbus are being exceptionally agressive with pricing to keep the Series out of major airline fleets. United and easyJet being the two most often mentioned.
The investment in a new fleet would be huge - I agree, but I don't think the E-Jets are sustainable. easyJet and Ryanair started off with much smaller capacity aircraft. EZY's -300s sat 149 compared with their A320s with 186 seats, Ryanair will have 198 on the 200MAX - big aircraft for thinner routes. So their may be a gap forming, if frequency is desirable. Of course newer technology, denser aircraft will make the gulf in costs between the E195 even larger, so I do share your thought that the Q400 should remain the backbone of the future fleet.