I totally agree with your concerns about people being kept in their job for unjustified reasons. I think it's something we've all had experience with.

At my first job, I had a horrible boss. A friend tried to speak up against him and was promptly fired. Turns out he was a relative of the CEO and was thus unfireable.

Just a few weeks ago I also spoke to someone who told his experience of reporting someone for sexual harassment at a major tech company, just to learn that they were well aware of it but refused to do anything because of good performance.

We should absolutely consider the role uninions can play regarding unfirable employees. Being able to push the company to get rid of them can have a huge impact on the well-being of workers.