The problem for the union is that Verizon can play this game for months if they have to. Landline doesn't provide much revenue or (especially) profit to the company so they don't really care if they get a few mad customers who drop the service. The fallout would be a drop in the bucket to VZ's financials and it's a business they don't really want to be in anyway.
So from VZ's standpoint they can let the replacements/managers stay in the field for however long they need to. Bad service from the replacements? Who cares. Lose a few FIOS customers? Who cares. Slow trouble ticket resolution? Who cares.
VZ's entire landline business could go belly up tomorrow and VZ wouldn't care -- it's a drag on their overall financials anyway.
The people who will ultimately get hurt are the union employees because every customer who drops their service results in one step closer to laying off the union employees that are causing all the issues.