As a sub-national state with a large degree of autonomy, especially with regard to linguistic and cultural issues, I doubt that Quebec should have any fears with regards to assimilation, no matter what it's share of Canada's population is.
There is no question however that in the worst case scenario that you paint, that Quebec's influence on the national stage would diminish.
At the end of the day, how important is that too Quebecers? They will remain
Maitre chez nous in Quebec.
A decreasing French influence in Canada is more of a concern to francophone minorities elsewhere, even in NB where there is some degree of constitutional protection.
In the last census, the francophone population of NB dropped to a flat 30%. If international and interprovincial immigration continues on into the future to the same degree as right now, the francophone proportion of NBs population might drop to 27% in 10 years and 24% in 20 years. Even right now, Blaine Higgs was able to form a majority Conservative provincial government with virtually no francophone representation. This will only become easier and easier to do in the future.
Francophones might eventually become as irrelevant to NBs government as anglophones already are to Quebecs government.