when you have given 10 years of good work and are considered too expensive and your company give you redundancy notice and recruit someone to do your exact job at half the salary?
If they can find someone to do the exact same job at half the salary, I'm probably being overpaid - in which case, how nice for me to have had that for ten years. If they're taking a hit on performance to do that - well, the market and their competitors will determine whether my work is actually worth the rate I'm being paid.
Unions are very keen on pay differentials so (at least the ones I've been involved in are). So they will negotiate, say, a baseline salary, and then increments for promotions, length of service etc etc.
I don't like length of service payments, I think they encourage stagnant mediocrity. I've worked in places where you get annual increments by default and then very occasionally you might get a second increment in one year, or a promotion. But you definitely can't have that year on year if you perform significantly better than average (and of course there is variation). So why would anyone bother trying to be really valuable? IME a promotion either involves company politics or a change of job role and so that doesn't leave any scope for just rewarding people who are doing a really good job of what they're doing.
What I am saying is that I don't think everyone doing the SAME job, should necessarily be on the SAME salary.
Exactly. Not everyone doing the "same job" does the same amount of work to the same standard. Why should that not be reflected in peoples pay.