for once you're right. It won't be a 1 for 1 trade with 50/900. The growth for ASA will be the new 100 seat planes (no details on what it will be as of yet) along with the 900's.
Can't find enough pilots and the airports/sky are too crowded already (if you choose to believe the feds). So, the alternative is to move the same number of pax (or more) with the same number of crews already available (or fewer)....answer....planes with more seats.
Mainline will fly anything with more than 100 seats and "connection" will fly anything with fewer than 100 seats.....of course northing official has been announced, but remember you heard it here first.
That will be one hell of a trick since DAL scope prohibits you from flying anything with more than 76 seats.