The US will never take Canada by force, so I don't know why you're so fixated on that. If the US does take Canada, it will be due to internal strife with those 50% ruling over the other 50%, making their lives harder with poor policy that negatively affects them, and them wanting some sort of treaty with the US, breaking away internally.
There are only two ways that it can be done. One way is to encourage immigration, which is already showing its face (in the US too), making the lives of all Canadians tougher.
The second way is to incentivize the family unit and birth, something that Trump is floating already for the US.
Each way has its cons and is an investment for population growth. If the wrong population grows, it can stress the system too much. That said, without one or the other, the system is already primed to collapse as more people get to retirement age than enter the workforce.