And yet you've just generalised without looking deeper. "They fire all the young people" is a hot take, until you realise that young people will far more easily find a new job.
You inserted the "all", I did not write "they fire all the young people". It's also just an example.
It is also not a given that young people will have it easier to find a new job. Youth unemployment is at staggering heights in many countries.
And by your logic, there still is no incentive for young people to support the unions. They could just give up their jobs voluntarily, if they are so convinced that it is the right thing to do.