A country is not just a random set of people. They have to be linked by a common set of values. Importing people who don't want to assimilate, or importing more people that can be assimilated will lead to disaster in the long term.

Additionally, immigration is not a solution for low fertility. If they assimilate, they will also have a lower fertility rate, pushing the problem down the line. If they don't assimilate, and keep a high fertility rate, they will replace the population.

Immigration is a positive thing, but it needs to be low and open only to like minded people.