I've worked places that refuse to fire low performers and its hard for it to not be toxic. I'm not saying this outcome is a forgone conclusion of unions, but my union experience is that poor performers take even longer to get rid of and I'm not sure I would be interested in that sort of environment again. This is more of an implementation problem than philosophical, but theoretically good and practically bad is still just bad.

You don't have to fire "low performers" you just have to be realistic about their skillsets and use them that way.

If you see an engineer is out of his depth then change his position, no need to fire them since like others have pointed out, that can have severe consequences in their personal lives and most of the time they can still be useful if they get more adequate work.

Until healthcare and housing aren't tied to employment, making it easier to fire people will always be the monstrous position. If you want firing people with abandon to be socially acceptable then you need a public safety net. Until that's in place, I'm always going to be on the side of labor organizations demanding dignity than the people destroying lives.