It would being better pay, better benefits, even more stable careers and better work-life balance.
It doesn’t matter how much money you’re already making, or how good your benefits already are. If you have a Union, you can negotiate for improvements. There is always room for improvement, unless you’re working at a fully-mutual workers cooperative.
I know first hand that some trades even make more than their unionized counterparts
I’d be interested to learn more, do you have a source or anything?
This is the difference between a trade union and an industrial union. You can join an industrial union elsewhere in Europe or even in the US, such as the IWW.