Canada has about 1.8 million new vehicle sales per year. It's not impossible to serve a market that small, but a lot of profits in the auto industry are due to their ability to scale.
Any new manufacturer will have to start in the high priced, low volume, luxury segment anyway, but there isn't huge room for growth while remaining in Canada.
If they expand to the States, then they just end up with the same problems we have now. If they expand to Europe, shipping is a pain, though doable. But if that's the plan, anyone with enough money to start a new car company will probably just start it in Europe to begin with, since Europe has a bigger market than Canada.
The other way to do this would be a non-profit or Crown corporation, where profitability isn't the goal. That has a lot of other issues, but avoids the biggest one.
Are you under the impression that this new manufacturer will somehow capture the entire market?
To use Tesla as an example, since they're the largest and fastest growing "new" auto manufacturer... In 2024, they sold about 50,000 cars in Canada, and manufactured 1.7 million. So we're barely 3% of their market, and if their Canadian sales drop to zero (as they should), they would barely notice.
As you said, licensing could save a lot of R&D costs, but it would almost certainly come with a stipulation that we couldn't sell the vehicles outside of Canada. If a new manufacturer were to take up the entire Tesla market in Canada (which would be incredibly ambitious), they'd need to be about a quarter (or less) of the size of the Oakville Ford plant. I don't think that it can be profitable at that scale, but I'd love for someone to prove me wrong.