I disagree. Learning supposedly useless subjects like maths and art etc seem to help in wiring up neurons in your teenage brain to be able to unconsciously think or see better than otherwise.

It isn't the specific details of the math subject matter that is the benefit - that stuff is forgotten. I don't directly use any math techniques in my day to day life, but I can appreciate how a few years of learning it and practising solving problems in things like geometry, trig and calculus has given me a really intuitive feel for quantities, spaces and their changing relationships etc that I wouldn't have had without it.

Long term, learning is less about retaining specifics and more about training your brain to be able to learn stuff. Different subjects can exercise different pathways and it's all potentially valuable.

Your conscious mind may have forgotten all the specifics, but your unconscious mind has been improved by that practice and given your intuition a better starting point when solving new problems. And you may not really be aware of it.

No learning is completely useless. Although some teaching can be completely useless.