One of the worst things about using immigration as a solution is that it's merely a bandaid. The worst thing is that people who encourage mass immigration are (likely) unknowingly saying their country is more deserving of the temporary benefits of immigration, and the source country can go to hell.
Many countries that are currently sources of large numbers of immigrants are rapidly approaching sub-replacement levels (e.g. Mexico), if they haven't already reached that point (e.g., Vietnam, China). The idea behind mass immigration is that you have a constant supply of workers and tax payers to subsidize the welfare of the retiring natives of a country. The idea of having a class of people subsidize others is troublesome enough, but the real problem is those less wealthy countries that immigrants come from are now lacking in workers and losing their own future. For countries that are still only beginning to develop, such as Vietnam, their future is a hell of a lot more at risk than something like Canada.
First world countries relying on immigrants have infrastructure and various frameworks built up that'll last them for quite a while after populations begin to dip. Poor countries do not have that benefit. Shit will hit the fan and hard. Young people who struggle to meet basic daily needs are going to be pissed when they're working for dollars a day and expected to support the social nets given to 3 generations above them while being told there's nothing left for them.
Right now, the government is saying we should mass import Vietnamese people to change the diapers of our old and do dirty construction jobs. What's Vietnam going to do in 25 years when its population crisis catches up with it, and every country around it is going through the same problem? Nobody's changing their diapers and their streets are going to crumble from insufficient laborers.