Sorry, that's not my assumption. It's a policy position taken by every Western nation. An increasing population is healthier for a country than is no population growth. The baby-boomer example I provided is just one of many possible scenarios that make no population growth a dangerous situation for any country. Anyway, I answered your question: why have any immigration at all?

Now you want to shift the goal posts to have "targeted immigration policies." In that case you'll be happy to know that Canada has never permitted mass migration. Since at least the 1960's Canada has targeted specific types of immigrants, namely immigrants with an education, a skill-set or financial assets that we can put to good use. But what Canada doesn't do is demand that immigrants must be white Christians as well. If you have the qualifications as above, you can immigrate to Canada regardless of your race, religion, ethnicity or nationality.