Is a math degree really useless financially?
Just a personal anecdote, not really an answer, too long for a comment.

I have a master degree in mathematics and now I'm doing physics simulations for visual effects in Hollywood blockbusters. I believe that a math degree gives you a massive upper hand in any field with reasonable amount mathematics in it. However, you have to be interested in that field and study it in your spare time. I studied computer graphics in my spare time and my math background allowed me to comprehend things in computer graphics orders of magnitudes faster and deeper compared to fellow computer science students.

Take it with a grain of salt, master degree is completely different kind of a beast compared to a PhD degree.