50 seaters were drawn up pre-9/11 not to replace mainline jobs but to steal passengers from other carriers. Example, Erie PA (connecting to PIT) was a US dominated airport but with the RJ Delta, United, Continental, NWA etc. could steal passengers from them and get them to their hubs.
9/11 happened shortly after the RJ build up and that was consequently when passenger traffic fell off the cliff. RJ's were used to keep service while mainline airplanes were parked.
The oil runup changed the 50 seater again and made it a loss leader, until airlines began flying them at M.60 instead of 300+/.74. Airways said at that fuel burn they were making money off them in DCA.
However by 2008 they'd gotten to the 90 seat range and management has realized they might be able to start actually replacing mainline airplanes with their subcontractors.
After the large mergers the "big 3" suddenly had much more passenger traffic (think a NWA and DAL CRJ departing going to their hubs at the same time) where now a 100 seater can become the new RJ. United is looking at ordering E190 or C-Series aircraft, DAL bought 717/MD80's and who knows what AA is going to do, they have other issues on their plate according to Kirby for the time being. It hasn't even been a year since the companies merged. If mainline can hold the line on scope I personally think we'll see a reduction in aircraft as they are up gauged at mainline partly due to lack of pilots and partly due to 100 seaters being used as RJs.