The issue is that if you import all of skilled workers you need, salaries won't rise in the branch, and students or workers in adjacent fields don't have any incentives to learn the skill. If you can hire Syldavian photonics engineers for 50k$/y, no one is incentivized to learn photonics.

This is why in the modern world some sectors are always crying about "worker shortages" while asking always more migrant workers to come in - salaries are compressed as a result, and locals have no incentives to enter the branch.

Hence importing workers may be useful for needs that are limited in time and space (say, install a specialized foreign machinery in a power plant), but systematic importation leads to dequalification of the local workers, who flee the branch that has now low salaries.