How migration brain drain impact the countries of migrants’ origin?

That's too generic a question as it can very much depend on the country and the nature of the migration. Reactive an circumstance/event driven migration does not always result in a brain drain for the country that the migrants are leaving.

Those that feel that they have less chance of progression in their home country - possibly because middle management roles and above are allocated through nepotistic, oligarical or corrupt processes can be a more significant loss.

Consideraton needs to be given too about whether the migration is of a temporary or permanent nature. Whilst a brain drain could be an issue there could also be a net gain for the migrant's country of origion if an equally trained or qualified person replaces the migrant from a perceived Economically weaker country