Well, the question is why those unions are strong in legacy carriers? In my outfit there was no union recognition until around 7 years ago. The pay was extremely bad, hiring DEC was common and using TREs to get rid of unwanted elements in the pilot work force (by "checking them out [of the company]") was normal business.
Then the union got hold in the company, mainly because two unionized companies were bought and integrated. Seniority was instated, the hiring of DECs stopped, nobody can be checked out of a position, he has to be trained up to standard again if he fails a check, and well, pay more than doubled in that time period. Entry level FO pay (type rating paid by the company of course) is now what used to be entry to mid level captains pay. Since seniority and union systems now mean that there is a viable career in that carrier, pilots want to stay with the company instead of moving on after minimum time and pressure is high to have better T&Cs.
Not one of the pilots in the company has asked its company council to negotiate rules that allow hiring of DECs in front of its 15.000k+ hour FOs, quite the opposite really. Since the CLAs still have rules where to put DECs on the payscale based on prior experience there were many demanding that those rules where to be scrapped since we do not want any DECs anyway. As the company wanted those old rules to be kept the company council decided not to press the point, especially as it was easier in return to increase the T&Cs and no DEC can be hired anyway unless there is no qualified FO left in the company.