Yeah, you handwave some of the most visible and effective unions in the United States (SAG-AFTRA, WGA), you're also ignoring education unions, healthcare unions, and many other careers where pay is not just about seniority, but also education, certification, specialization, and other factors.

Even unions in industries with legitimately undifferentiated labor will have shift differentials built into their contracts.

No, it’s recognizable and sports unions often refuse to break picket lines for low wage workers:

https://amp.theguardian.com/football/blog/2023/sep/20/inter-...

What you’re doing is trying to craft a special definition of a union that excludes the most analogous unions to the one that software engineers could form and ignores the real examples of solidarity between professional class and other types of workers. See also the solidarity strikes against Tesla in Sweden. I won’t speak to your motivations in attempting to do so, but it doesn’t correspond to how labor organizing is actually practiced.

I would put a hypothetical software engineer's union in the same category as a sports union/actor's guild. Just as an actor has brand recognition and can leverage this or start their own productions, so can a skilled software engineer start their own company/be recognised in their craft. And beyond that, can you imagine what would happen if software engineers decided to strike? It has the potential to have a far greater impact than pretty much any other industrial action. If factory workers down tools for a week, production is delayed. If Google SREs down tools for a week, it might break the internet.