> In a union, the only way up is seniority

This isn't a strict requirement of unions though, right? As a trivial example grad student unions have no real career ladder, though the union negotiates a minimum pay and amount of work for everyone.

> the only way up is seniority

That’s not true at all. Look at professional athletes. The starting pitchers in a baseball game are the best pitchers. Or consider WGA screenwriters in Hollywood. Their ability to make money doesn’t depend in seniority.

The typical carreer ladder for most people on a game dev team is basically to get fired when a game project ends and trying to get hired by another game company that's just starting a new project ;)

Maybe exaggerating a bit, but that's the reality in many game dev shops, especially when a game doesn't immediately sell in great numbers.

Unions have nothing to do with career progression…