Realistically collective bargaining leads to a dual effect of the median salary rising while at the same time more people will end up earning closer to that new median. So it's hard to put a percentage on how many will 'suffer'. But that fear of not being rewarded for your perceived potential is very real.

Secondly, and perhaps more importantly, collective bargaining is about more than just salary. Non salary aspects like paid vacation days, generous sick leave policy, paid overtime, maternity/paternity leave policy etc. etc. can also positively effect people even if their salary doesn't increase.