Controlled economic migration is fine as long as it keeps finding the jobs there.

It turns into disaster when you mix it up with social welfare, dei culture, other curtailment of meritocracy, drop any entry limits, ..., and despite seeing that certain groups of migrants aren't finding the jobs, you keep funnelling it in.

Maybe the root cause behind historic populations dying out needs to be looked at and treated instead of just patching it over, and fixing the symptom represented by overall population volumes.