1. I am automatically sceptical of any pro-immigration arguments that involve staving off population decline. Population decline due to low fertility is going to be global problem in the near future, including in the countries where the majority of our immigrants come from (India, China, Phillippines, etc.), so what you're doing by mass importing people here is delaying the population decline temporarily while shifting the burden of the population decline from the economies of those countries to the expensive and largely outdated Canadian welfare state.
2. Canada has 350,000 births and 330,000 deaths annually, as well as an emigration rate of about 100,000 per year. This adds up to a total population loss of about 80,000 per year, so we do not need more than 300,000 people a year to maintain our population.
3. Even when we took in 250,000 permanent residents annually for decades, our population growth was one of the highest in the world. I highly doubt that reducing that number slightly is going to make our population fall. According to several economists at the National Bank of Canada, we need our population growth to be below 500,000 annually to get out of the 'population trap' situation we are currently in. FTR, when both permanent and temporary residents are included, our aggregate population growth has been over 500,000 annually every year since 2018 (minus 2020, for obvious reasons). We are already saturated and don't have the jobs, housing, infrastructure, etc. to accommodate all the people currently here.
4. If the cost of increasing our population is importing cheap labour to depress our wages, saturate our infrastructure and subsequently reduce our standard of living to third world levels, maybe increasing our population by that much isn't such a good idea.