OK, maybe it was an exaggeration to say that Canada's economy will collapse. However, ti will be seriously inconvenienced. Most of the immigrants who come to Canada are professionals. Doctors, engineers, scientists etc. It is very expensive to train them locally, we get them ready made from third world countries.
Immigrants bring in skills much in demand in Canada. Without those skills, Canada will be at a disadvantage.
To imply the entire economy will be seriously inconveneiened is a bit much too. Capital will be very modestly inconvenienced if immigration levels are lowered. Labour, which is most of us, will benefit because of less competition for jobs. Wages will rise, yay. Business will have to be more productive, so they will pay more, yay. Then there might be fewer jobs, but with less immigration there will be less competition for those jobs, yay. So business will have to be nicer to workers to be able to hire them, yay.
I only see great benefits for labour with lower immigration levels. Let capital be very competitive, they are very very smart. But since we can control who lives in Canada, let's do things for the benefit of labour, the wage slaves, which is most of us.
Most immigrants are family class and many of them are uneducated unskilled people who take menial jobs and speed the race to the bottom. Not no immigration, but lower levels.