While it is true that some rural seniors will follow their children to the city, this is not universally the case. Many will stay in their home communities. This is what they are familiar with and it is where their friends are. As long as seniors are able to stay active and independent they will stay where they consider their home to be.
I do not predict a northern "tsunami" hitting Moncton, but I do predict continual immigration from the north as well as from elsewhere in the Maritimes and Canada, even overseas.
Only about 30-40% of immigrants to Moncton are from northern NB.
A similar number is from elsewhere in the Maritimes, and although you might find it surprising, about 20% is from the rest of Canada. There are also several hundred foreign immigrants to greater Moncton each year.