Well, 76 seater's are now effectively capped on the NWA side with the JPWA, so there is only so much revenue they are going to contribute. The 50 seater's are toast, and even if they have better costs, still only carry 50 pax, are frequently weight limited, and cancel with regularity due to staffing shortages.
Managements are also getting fed up with the endless delays caused by too many RJ's flying into the likes if LGA, EWR, JFK, etc. - it still takes up the same amount of space in the ATC system.
There is a niche for this aircraft that there is nothing else that fills for the *moment*, and despite the supposed economics of the 76 seater, however pax still
hate the cabins, and management is growing weary of the the feeder operation reliability and quality issues.
The DC9 (and MD80's) will be the first to go, however I think reports of there their death are premature. Also consider there are alternatives for the 142 seat MD's now (A319/320, 737NG), where there is nothing for the 100-124 seat DC9 market. Despite your infatuation with the 76 seater, they are an inferior product from every perspective other than SJS.
I've been wrong before (once

), but I'll wager the DC9's are around until a suitable, true NB cabin, mainline operated NG 100 seat replacement is deployed.