There is also, in my opinion, great benefit to just training your brain. Even if you forget all the details, I think someone who learned calculus and then forgot it is going to generally be in a better place to handle any general mathematics than someone equal who never learned calculus at all.

Sometimes going a few steps deeper helps you retain some useful bit of info. I ended up making useful of various bits of chemistry decades after taking the class, but understanding the basics was still helpful.

Finally basing things on utility is just a prediction. Some things that seemed useful turn out less so as technology filled in a gap.

The person who would fail that math or French conversation right now, would be able to get up to speed in days, weeks or months depending on what exactly you want if needed. However, learning French from nothing would take years again.