The union can force an all-or-nothing decision, but some companies can and will easily choose "nothing" and keep only the overseas developers, not the local unionized ones.

I think there would be too many programmers who would be proud to take the jobs of other programmers trying to unionize.

There are a lot of anti-unionists and Libertarian minded programmers in the US.

Libertarians aren't anti-union, they're anti-government enforced unions.

Unions are meaningless with collective bargaining enforced by the government.

Then they just hire consultants for overseas training.

That would certainly make the union workers more attractive given the average consultant’s experience, teaching ability, and understanding of the business.