The bottom 50 probably don’t benefit from math education anyway.

I seriously doubt this. There are some many places in life where a little bit of simple mathematical thinking would/could go a long way and you don't need to be particularly bright to understand a lot of it. Almost anyone can learn how to think about different orders of magnitudes of numbers through visual representation, but it just isn't taught very well. The difference between a million, billion, and trillion dollars matters a lot in civic life, and parts per billion are useful for understanding day to day health risks. Anyone can learn that.

No, everyone benefit from higher education from pure production/productivity standpoint. Whether it contributes toward GDP and/or QoL are different issues.