And those 20% won't even be compensated 2-3x higher under a union: likely just in lockstep with their years of service. A union might figure out a couple of tiers within the payscale for specialized or managerial work, but I expect those to be gamed as well by shrewd but mediocre people.

Following up on what you say, the main reason for unions is to help labor extract more of the wealth they create. While I'm sure there are lots of people here who think devs should make more of it, they live pretty comfortably. It'd be like saying doctors should unionize to get a lockstep salary from the hospitals where they work. There might be some marginal benefits to it, no doubt, but they don't as a class seem any more interested in it than devs do.