What is the possibility that the Regionals will become too expensive to operate and mainline will just replace 2-3 RJs with B737s and increase tickets prices a few dollars to increase pay for turbo prop FOs???

It will be a huge test to CFI in a C172 for 1500 hours. A test of skill and will.

Could this be the end of the RJ operators if a shortage/cost have a big enough spread?

Probably a pipe dream, but it would be a dream come true.