No, it's much more simple than that. Developers can simply cut their apps from the ecosystem for a short while. Or move to another platform in large numbers. Just the threat of each action should cause those companies to consider some concessions.
You're missing the point. Individual workers or developers are replaceable, and at a serious power disadvantage. A union works by blocking all access to labor, and requiring the company to negotiate with the union alone. Unions used to do this by physically blocking non-union workers from going to job sites, today there are formal legal processes. There is no alternative to either method available to developers for closed platforms. If you opt out of developing for the more lucrative platforms, there are plenty of people willing to replace you.
I know it's more complicated than that and that there are exceptions, and that over time some places become hybridized. I'm still right.
