Trade unions tend to have a large focus on training and skill building. Yes they make firing people harder but the flip side of unions can be increasing the average quality such that firing due to quality or skill is less often necessary.
If I have need 10 people and I have a choice between two looks. Pool A is harder to fire and costs more but has a 95% competence rate. Pool B is easy to fire and costs less but has a 70% competence rate.
Setting any contractual rules aside I’m going g to find myself firing from pool B more often, and it is easy to attribute all of that to the rules that make A harder to fire. They are easier to fire (component of variance), they are more likely to be bad (component 2), and because they are easier to fire in going to be more likely to fire than coach for quality (component 3).