The auto business was enormously profitable for GM and Ford, but there were thousands of equally promising auto startups at the same time. Most of them built prototypes bespoke for rich people; none of them are still around.
[1] And cars. They used to own about a fourth of GM, until they were forced to divest.
I think it is better to think of labor unions themselves as startups that eventually consolidated. They gained control of a resource that was in demand by a growing industry, manufacturing. They grew by competition and consolidation (as did their most successful customers) They supplied that resource at a price negotiated with their customers. Manufacturing prospered, and so did it's suppliers, including the labor unions.
The labor unions failed to adapt to a changing market (as did many of their customers), which is why they are on hard times.
That they failed to adapt says little or nothing about the considerable, commitment and sacrifice it took to found them. I agree though, that the presence or absence of those attributes in modern day union organization and membership isn't the whole story about why they are throughly failing today.
They don't make something people want. They take something people want and jack up the price to make them pay more for it. They're as much of a startup as OPEC -- the only difference is that they're cartelizing labor instead of oil.