You say this as though regional pilots WANT mainline to shrink, when nothing could be further from the truth!

Regional pilots WANT legacy pilots to quit selling scope and giving away increasingly larger "small jets", but if those airframes are sold by legacy pilots (as they have and continue to be) then can you really fault regional pilots for wanting to be the ones flying them?

There is PLENTY of room for a happy middle ground via negotiation and bridge-building, allowing for job protections and career advancement for pilots regardless of what size airplane they are currently flying.