The problem is that immigration ended up being almost entirely Anglo. And that's a really tough obstacle to overcome. The fact that the French speaking population stayed as large a percentage as it did required an amazingly fertile Québec (La Revanche des Berceaux).
The jobs, land and other opportunities were in Anglo Canada, so any non-English speakers naturally assimilated to the Anglo population.
So... How can we fix this?
Firstly, OTL, there was a lot of Franco emigration to the US
https://en.wikipedia.org/wiki/French_Americans#New_England.2C_New_York_State said:
In the late 19th century, many Francophones arrived in New England from
Quebec and
New Brunswick to work in textile mill cities in New England. In the same period, Francophones from Quebec soon became a majority of the workers in the saw mill and logging camps in the
Adirondack Mountains and their foothills. Others sought opportunities for farming and other trades such as blacksmiths in Northern
New York State. By the mid-20th century French Americans comprised 30 percent of Maine's population. Some migrants became
lumberjacks but most concentrated in industrialized areas and into enclaves known as "
Little Canadas."
[17]
=
The
Statue of Liberty is a gift from the French people in memory of the
American Declaration of Independence.
French Canadian women saw New England as a place of opportunity and possibility where they could create economic alternatives for themselves distinct from the expectations of their farm families in Canada. By the early 20th century some saw temporary migration to the United States to work as a rite of passage and a time of self-discovery and self-reliance. Most moved permanently to the United States, using the inexpensive railroad system to visit Quebec from time to time. When these women did marry, they had fewer children with longer intervals between children than their Canadian counterparts. Some women never married, and oral accounts suggest that self-reliance and economic independence were important reasons for choosing work over marriage and motherhood. These women conformed to traditional gender ideals in order to retain their 'Canadienne' cultural identity, but they also redefined these roles in ways that provided them increased independence in their roles as wives and mothers.
[18][19] The French Americans became active in the Catholic Church where they tried with little success to challenge its domination by Irish clerics.
[20] They founded such newspapers as 'Le Messager' and 'La Justice.' The first hospital in Lewiston, Maine, became a reality in 1889 when the Sisters of Charity of Montreal, the 'Grey Nuns,' opened the doors of the Asylum of Our Lady of Lourdes. This hospital was central to the Grey Nuns' mission of providing social services for Lewiston's predominately French Canadian mill workers. The Grey Nuns struggled to establish their institution despite meager financial resources, language barriers, and opposition from the established medical community.
[21] Immigration dwindled after World War I.
To help keep the French population in Canada up, one obvious way would be to provide more jobs IN Canada, have people move to e.g. mining towns in Ontario, or to farms out West, instead of moving to the States.
This has multiple effects. 1) it keeps those people in Canada. 2) it helps keep the birthrate up (more on this later), and 3) if e.g. Manitoba stays predominantly French speaking, with more Québecois moving west than Orange Lodge Ontarians, then even if 'peasants in sheepskin coats' DO come to the prairies later, they might well speak French rather than English.
OTOH, that's going to be tough. It was a lot easier to get from Québec to New England mills than it would be to go to the prairies - as there's no railway west yet.
You'd need a deliberate effort to populate the West with anyone who'd go, rather than leaving it up to individual initiative. Ontarians had been hacking farms out of the bush for most of a hundred years - plowing the prairies is easy by comparison. Whereas the Québecois had pretty thoroughly cultivated the land available, so young men going west would not be used to hacking a new farm from the wilderness. Thus there needs to be stronger incentives for the Québecois, IMO. And those incentives would be very unlikely to come from Ottawa. Maybe the Catholic Church needs to proactively encourage farmers to go West, providing hundreds of local priests for the necessary churches, as well as the seed money needed to start them going.
Get a few thousand French speaking Catholic settlers to Manitoba by 1850, say, and you could make the Prairies fairly solidly French speaking... (The more people there early, the more you can send later.)
And, again, having 10 kids (say) when you have a postage stamp sized farm on the St. Lawrence means it's really difficult to support them all - especially when it comes time for THEM to inherit and have 10 kids each. Having a 'relief valve' out west would help a lot. Farms can use just about all the labour they can get from their kids, and having the entire Prairies available to homestead new farms means you don't have to keep subdividing a farm that was subsistence to start with.
However, this also means that SOME sort of railway has to happen earlier than OTL. Building rail from Winnipeg west is not a problem, of course, as the land is flat and solid. OTOH, the OTL TransContinental Railway (CPR) that ran north of Lake Superior was an incredible sink of resources and especially money. My guess is what happens is that rail is run to Lakehead (Port Arthur), and grain is shipped out via ship from there, and supplies are shipped in. That means that the prairies are essentially isolated from the rest of Canada for, say, 3 months of the year, but ...
I would also guess that a post road would be hacked out for e.g. mail north of Lake Superior, but it wouldn't be enough to carry freight or an army (either for putting down a hypothetical rebellion, or for defence against US encroachment).
How's that?