A CEO can be replaced. Management styles can change. Why would a union see a short term boon (good relations with management) as a reason to sacrifice long term

A CEO can be replaced. Management styles can change. Why would a union see a short term boon (good relations with management) as a reason to sacrifice long term goals (best working conditions for their members)? Also, there's no such thing as a non-adversarial company that would require a union and vice versa. If you're building an employee-owned structure (or similar structure in which all stake holders are able to help sculpt company decisions), you don't need a union. And if you don't have employee ownership, you're not giving your employees a real seat at the table and thus don't represent their best interests.