What do mathematics researchers do if they aren't good?
Many people who complete PhD's in mathematics end up leaving academia within a few years after completing the degree.  Many others settle into teaching oriented positions at community colleges, four year colleges and regional comprehensive universities where they typically end up publishing little or no research.  A small percentage of all PhD's in mathematics end up as tenured faculty in research universities (much less than 20%) and even among these mathematicians at research universities there is huge variability in research productivity (e.g. as measured by papers published per year) and impact (e.g. as measured by citations of these papers.)