How then would a union ensure that developers aren't all fired? Game development companies have a lot of experience hiring and laying off large numbers of employees in one go, that's what they do when they start or finish developing a game, so I suspect the difficulty of replacing large numbers on short notice wouldn't be as big a deal as you make it out to be.

Also, I suspect there is a significant difference between being easily replaced because the job requires minimal training and being easily replaced because highly qualified people are willing to forgo better paying, less demanding jobs in a similar industry because they've spent their whole lives wanting to make games.