There are two points to consider:
1. Legal clauses that specify retaining top performers. It is best practice for a buyer to retain the bets performers from the target, hence why HR due diligence is key. So a smart buyer will put in clauses that ensures that they stay in the company (people leaving cannot pouch them and provide stock options)
2. Ethics, the buyer paid what should a fair price to the target. If taking great talent will harm the buyer then it is not ethical to do so.
In the end M&As have a harsh impact on employees but in some cases can help the greater good by sustaining failing companies from going under completely.
Be safe everyone!