I would say there are certain cities, probably lots of them, that only support a partially full 50 seater. When DAL dumps the 50 seaters (and they will, regardless of whether we pass POS2012), they can do one of two things.
A) If it was a mostly full 50 seater, maybe the market will support a 70 seater (No increased mainline flying).
B) if it was a partially full 50 seater, it's probably not worth putting a 70 seater in the route. Drop the market altogether (No increased mainline flying).
That's why I think the 70-76 seaters are so much more dangerous to our jobs. THEY are the jets that replace our flying. Try to ignore the distraction put out by ALPA regarding 50 seaters and overall numbers of RJs. Look at what happens to the number of MAINLINE REPLACEMENT JETS (70-76 seats). That's what we care about.