The union defends their members, that's part of their job. It's what prevents administrators from arbitrarily firing people they don't like - or, worse, people who stand up against malpractice on the administrative side. The administration has access to their own specialists who can and do "fight tooth and nail" for their side.
When someone is actually incompetent any halfway competent administrator will be able to fire them. It's not hard - it just requires collecting some actual evidence and documenting that you followed the processes correctly.
Which would you rather have, a teacher that taught and inspired your 6 year old kids, or a generic licensed teacher?