While American Eagle is opening new routes with the ERJ-145, they are also using them to replace current Eagle service (i.e. ORD-IND) and to augment current AA service (i.e. ORD-ISP). There are no markets in which American Eagle has replaced mainline American flying that I'm aware of. If you know of one, what is it? American did not fly ORD-BWI prior to Eagle beginning service, nor did they fly ORD-PIT. If those could truly be operated as mainline routes, Eagle would not be able to satisfy the demand using 50 seat jets. Eagle currently flies 5 time a day ORD-BWI, for a total of only 250 seats. For American to operate this capacity, they would have to reduce to 2 flights a day, and severly limit the convenience to passengers. Flights currently depart from 615 am to 830 pm, with an average of 3-4 hours between departures. Who knows if the demand is even there at any one time for American to fill a 139 seat aircraft? As for Pittsburg, Eagle currently flies there 4 times a day, only 200 seats, again spread out over the whole day. Perhaps the time will come when AMR has established itself enough in these markets to commence mainline service, but until there is sufficient demand, the RJs make more sense.
By the way, some people learn to fly out of love for it, not to get the fat paycheck. While it would be nice to bring home a six figure salary, I would fly for less. As long as I'm making enough to get by, and I'm still getting to do what I love, I'd be happy. I don't think that any of Eagle's RJ pilots are new enough to be making $18,000 a year.
One last thing - I believe that most of the reason that Eagle does so much flying out of O'Hare comes down to slots. United, with their wealth of mainline slots, has shown us that routes such as ORD-IND can be done with large planes, American just doesn't have the slots. Eagle has them, so Eagle gets to use them.