ORD is historically amongst the most junior of bases, so time on reserve tends to be the least. Right now, staffing means that reserves get used the majority of their days on. I don't see that ending anytime soon. The E175 is a giant unknown, since less than a handful of destinations have been announced, and trip quality is in limbo. That will change with time, but as others have said, as they move West and more is known, expect it to go more senior. I doubt very much to see newhires in it anytime soon, they are much more likely to drop seatlocks first and let it go to proven entities as the Feds will be watching us very closely for quite awhile.

Incidentally, CRJ pilots will continue to have access to Delta benefits, and AA. E175 pilots will be United-dedicated, and the travel center recently announced that E175 pilots will have access to Delta benefits only as far as SkyWest-operated Delta Connection flight are concerned. No mainline access, no other partners. It may not seem like much, but that's a pretty big difference.