> The employer is incentivized to hold these back in any case, absent some countervailing pressure
The countervailing pressure is that non-unionized employees are more likely to leave.
Most unions in the US place a high value on seniority. The more seniority you have, the more benefits you get from a union. Good for people who have been there for 10 years, bad for new employees looking to get jobs. This makes unionized employees less incentivized to leave and more incentivized to stay and strike. Leaving (even if for higher pay) would sacrifice that union seniority that many members value.
The problem with Boeing is that there are not many aerospace employers to got to. An aerospace engineer can probably only work for 2 or 3 aerospace companies in their career. Not to mention they may have to completely uproot their life on each change of employer.
this isn't really true. To change companies you might need to move cities, but there are a ton of different companies that require aerospace engineers:
SpaceX
Blue Origin
Space Systems Loral
JPL
Boeing
Embraer
Airbus
Cessna
Northrup Grumman
Lockheed Martin
General Electric
Raytheon
Boom
Rolls Royce,
Honeywell
Eurocopter
Bombardier
not to mention the adjacent fields of
Green tech (wind turbines)
Drones/UAVs
And all the vendors and suppliers that supply to these companies. The number of suppliers very well would exceed into thousands of small and midsized companies scattered around US.
The countervailing pressure is that non-unionized employees are more likely to leave.
Most unions in the US place a high value on seniority. The more seniority you have, the more benefits you get from a union. Good for people who have been there for 10 years, bad for new employees looking to get jobs. This makes unionized employees less incentivized to leave and more incentivized to stay and strike. Leaving (even if for higher pay) would sacrifice that union seniority that many members value.
It's a very different negotiating dynamic.