> 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.

It's a very different negotiating dynamic.

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

There are a lot of options out there

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.