> A union would keep salaries high by not keeping the gates open

That’s the same argument people use against immigration.

I realize HN is a diverse place and not everybody has the same opinion on these matters, but I find it interesting that people here generally support labor protectionism when it applies to high-income earners like software engineers, but they don’t support closing the borders in order to increase the wages of low-income earners.

USA-centric (but not in the south): I'm pretty open to most immigration. The "close our borders" discussion tends to lump in skilled-worker programs; I think those are usually more amenable to most than general immigration. Regardless, my country was built on immigration and I think it'd be foolish to lose sight of that.

That's because everyone is a liberal until their job or salary is on the line.

This is what I'd consider the "traditional" answer to the function of a union. I think it makes plenty of sense.

The parent comment seemed to me to suggest that a union would both keep the door open and the price high, and I don't see any clear way a union would do that.

Additionally, the low hanging fruit for automation is going to diminish. We'll always have things to automate, but in general those things will decrease in value.