It is true DC9s are dirt cheap to operate but they are aging and hence breaks down more often. I commute to MSP and I see it often. Very frustrating. DC9 nonetheless, as you have indicated, cannot be replaced with RJs. For one thing, RJs are no longer economically viable, especially 50 seat RJs. Even CRJ900s and E175 are not that economical and they carry too few seats. Bottom line, I suspect more than few regionals will be gone in the next coming year or so.
All the more reason for you guys who are at the majors to stay firm with a lockdown scope clause. I do not want to see more domestic routes bled out to regionals. Domestic routes flown by DC9s and A319s or B737 should stay as mainline routes with mainline jets flying at mainline pay. I am not planning to stay at a regional at regional pay flying overstretched RJs. Scope Clause will improve my chance of moving up to majors.