I honestly don't think MTA depends too much on Monctonians to fill out their student body.
If I was to guess, I would think no more than 20% of MTA enrollment is local from the Moncton area. The university is just a touch too far to commute (especially in the wintertime), and, there are no public transit options between Moncton and Sackville. Moncton students really would have to live in residence or stay in a Sackville apartment, and, if you have to live on campus, then why not venture a little further afield to UNB, DAL, SFX or Acadia.
MTA markets itself as a small (Canadian equivalent) Ivy League caliber school and sees a lot of enrollment from NS, PEI and Ontario (as well as overseas).
The presence of MTA does not obviate a need for a UNB campus in the city of Moncton proper. It is an apples to oranges sort of thing.