It would also make mainline flying more expensive. If you fly an RJ on a mainline seniority list/number for, say, 5 years and then move to guppy or bus FO, now the major is paying you 5th year pay for your first year flying the the bus/guppy. The major looses the benefit/opportunity to have "cheap" 1st,2nd,3rd...etc. year labor broken down by airframe because they let you accrue seniority while flying an RJ. And we all know how airlines like cheap labor....

It's just business 101, In a perfect world in managements' eyes, they would probably want to send all new hires to the wide bodies 747,777 because they generate the most revenue and have the largest profit margins. And, send the senior people to the smaller guppies/bus' where the pay scales are lower= keeping costs at their lowest.