So dealing with a stagnating to slightly decreasing population will happen at some point, anyway. It's just that by kicking the can down the road by encouraging immigration for short term economic "gains" we will also have destroyed our identities, cultures, and people.
Europe has probably passed the point of no return at this point.
Given that robots cannot give birth to children today (or in the near future) and cannot go to war, I don’t see what is a viable option for those countries that want to close their borders today to protect their “purity”.