I said 50 years, so that's 1810. 10 years matter given the super high reproductive rates of the French Canadians.
In 1760, there were 66,000 French Canadians in today's Québec. In 1810 (without arrivals from France) there were roughly 240,000 French Canadians in today's Québec. So that's a massive increase in 50 years even without immigration. But in 1760, it's usually little known, there were also 7,000 French people living in Upper Canada, or Pays d'en Haut as it was called (Ontario, Detroit and Chicago areas). So without immigration, all these people combined would have gone from 71,000 in 1760 to 260,000 in 1810.
But if Canada had remained French 50 more years, one assumes there would also have been some immigration from France. This very interesting article by J.N. Biraben shows that migration from France to Canada accelerated in the last 2 decades before British conquest, as Canada was finally taking off and so attracting more and more people from France:
https://www.persee.fr/doc/adh_0066-2062_1967_num_1966_1_927
Between 1740 and 1759 (which in fact means between 1742 and 1754, because in the other years there was war and it was impossible to reach Canada from France), nearly 3,600 French people settled in Canada (definitely, not temporarily as was the case for soldiers and administrators). This is the highest number of French people Canada attracted in 150 years.
So the French population of Canada in 1810 would have been higher than 260,000. 300,000 seems like a very minimum, but if immigration had ballooned as it was starting to do, 500,000 people in 1810 doesn't seem too far fetched (especially considering that if the French Revolution had happened as it did, lots of people would have fled to French Canada).
The British were unable to populate and outnumber 66,000 French Canadians in Québec, so if only 65,000 French people lived in Upper Canada in 1810 (which doesn't seem too far fetched with at a minimum of 300,000 French people in Canada in 1810 and 0 British people), then it's game over for the British in Upper Canada (unless they deport everybody, but they didn't dare to do it in Lower Canada, so it's hard to imagine they would have done it in Upper Canada considering the numbers, and also considering that things that were still ok to do in the 1740s were looked at with horror by more civilized Europeans in the 1810s).
From there it's not hard to imagine that Manitoba and then the rest of the west is populated by Francophone outflows from Upper Canada. If Upper Canada is not British in its population, it's hard to see how they can land in Québec and trek for thousands of miles to reach the western prairies while bypassing all the Francophone settlements in between.
So most likely the British population would have been confined to the Maritimes, or there would have been other events that we cannot even imagine (like decades-long wars between Francophone and Anglophone settlers on the Ohio-Indiana-Illinois border).
According to Biraben, more French people settled in Canada between 1740 and 1759 (i.e. in the 12 years between 1742 and 1754 in fact) than in the 60 years between 1680 and 1739.
France stupidly gave up Canada just when the colony was finally taking off. Of course there would never have been as many French immigrants as the British immigrants who populated the US, and French Canada would never have been as populated as the US, but 50 million Francophones in North America today from Gaspésie to the Pacific is not something that would be far-fetched. The southern border could also have been more southern, as Detroit, Chicago, St Louis could have remained Francophone, depending on the outcomes of the local wars that would inevitably have happened between settlers of both nations and historical rivals.