If our mathematicians are 10% better, we’re not going to have train drivers at all.

Train drivers will then move on to do more important work. You're always going to need other people if you want to progress at the fastest rate possible. Humans are social animals and all the great things we achieve are as a unit. An independent ant will never be able to compete with a colony.

They won’t though because there are only so many spots in higher educated fields. So you have the problem of baristas with phds and train drivers with advanced math degrees

Humans have never run out of work. By your reasoning trains themselves should have caused mass unemployment. With better tools we just do bigger stuff. This has been true for all of human history.

that's a problem of subsidized liberal education, not a problem of lack of education-based jobs

I think the argument is that improving the train drivers maths skills by 10% has little to no return for society, while the same cannot be said of the mathematician.

You might be right about that being the argument.

I disagree. I highly value a society of people with basic education. I think a lot of people do. It's one of the things most people are happy to pay taxes for.