The numbers obviously change based upon costs, but they are not necessarily going to replace 717s with RJs. Sometimes it is the other way around- Manchester, NH was initially started as an RJ route alternative to BOS. Eventually the loads became high enough to support MD's and they replaced the RJs. I can think of many cities I use to fly to in an ATR/CRJ that are now almost exclusively served by mainline (VPS/ECP come to mind). In some cases RJs did replace 717s/320s, but in many of those cases it is either the RJ or nothing as the route is too thin. Think Muscle Shoals, now exclusively served y Seaport, I think.