200s and 700s will still be around. The 700s are the only planes that can currently do some of the mountain airports such as Aspen. The 200s will definitely be around for awhile. United just granted Air wisconsin a contract for 50 200s and 15 options (this will replace mostly E145s).

The other two operators flying 700s for United are GoJet and Mesa. Once the remaining 700s are removed from United flying at SkyWest and the last of the E175s arrive for Republic, SkyWest, and Mesa, United will be scoped out on large RJs.

Of course, by the time you finish your training and get enough time to come to the 121 world, it will look completely different.