The thing is that an expanded New France with 500,000 settlers would of included what is now Ontario and even Michigan both areas great for agriculture and would of had high population growth.
The French in Quebec grew from 60,000 in 1760 to 200,000 by 1796. No that not include the loyalist who would of arrive around 1800.
Therefore New France with a steady small emigration of 1-2,000 a year would very likely have a population wAy over 500,000. Remember New France had very low spermatic immigration and in late 17th century stopped snd grew from 5,000 to 60,000 in 60 years.
More than likely this would have been their most successful strategy, one where France allows settlers to move down the Saint Lawrence and into the more fertile areas of Southern Ontario, and from there into the Great Lakes region and then southwards. It really would not have been a huge demographic burden on France, but the French Crown did not want any settlement West of Montreal and therefore the colony stagnated. By the mid-XIX century the lack of land meant that French-Canadians began migrating in droves to the mill towns of New England and to Michigan, Minnesota and Wisconsin.