Collective bargaining would not apply to any of these salaries. Collective bargaining as we usually understand it sets the lower bound for salaries in a certain field, think of it as field specific minimum wage. Indeed, some Nordic countries have no minimum wage at all because ~all fields are covered by collective bargaining -- regardless whether the employees are part of the union or not!

So no, pay would not be lowered. I don't understand where this kind of a misconception comes from. Collective bargaining does not mean that everyone gets the same salary. As I said earlier, you are free to negotiate a higher salary, and companies wanting to attract top talent will still have to compete through compensation.

In the top engineering companies, collective bargaining could be used to negotiate other perks, like paid leaves, hour banks where you collect all work time exceeding a regular work day, paid parental leaves, and/or whatever topics are important for the work force. I'm not the one to define that though.

Collective salary bargaining is efficient in fields where the workforce is highly interchangeable, and the workers' ability to produce higher value is limited. If factory workers, miners, or nurses are not unionized they are really placing themselves in a precarious position.