You are correct.

Brain drain removes the most ambitious, hardworking people from their countries.

And then the high levels of migration decimate social trust in the countries that get the immigrants.

It acts as a destabilising force. This is intentional.

Think of it from the perspective of someone trying to build a superstate out of independent nations. You have to weaken the authorities of power in your member states if you want to further compromise their sovereignty.

Migration does this on both sides. You put the smartest people in poorer states into much more competitive environments where they are unlikely to achieve significant power. Clearly it's difficult to measure the damage that causes. The Pareto principle (also known as the 80/20 rule, the law of the vital few, or the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes. Just from a purely gut instinct, I think this sort of migration is terribly destructive for the poorer states over the mid-long term.

And you simultaneously discredit the governments that allow this level of migration. Also they get to infiltrate some likely loyal voters into the richer countries.

It's pretty devious really. Whatever you think of the EU, they have to be applauded for their amazingly nefarious social engineering. They've literally managed to brand the commodification of cheap labour a freedom.

On the 'economic benefits of migration'. Of course when add more people to an economy it grows. Duh. This is so obvious. But is that growth really good for everyone? Increasing competition in various industries lowers profits and wages. And how much influence has foreign capital (not strictly just the EU) had on the housing industry for example (a great example of trickle down economics)?

And no one ever seems to include Brexit in all these economic benefit calculations. A direct consequence of this policy.

The great lie of free movement is that free trade of goods and services is dependant on free movement. Not in the internet age. You and I and anyone with an internet connection can buy and sell goods all over the world from their bedroom.

I didn't actually vote Leave. But the more I learn about the EU the more I'm turned against it. This is a mostly speculative post. But if you consider the motives and the results, pretty obvious.