Just because you do a maths degree it doesn't mean you'll work in finance. A maths degree could open up a lot more options. For example, there are people who went on to do data science, machine learning and data analytics in many industries, not just finance, because in maths you learn statistics and R which is useful for this type of career. Others moved on to do software development and software engineering.
You could do a PhD in maths if you want to write papers and if that's what you want to do.