Assuming models have complete advantage over human mathematicians in all areas of math (IMO not guaranteed in 15 years, but likely enough), this seems to be a question about the shape of society. Arguably the answer is not at all special to math--presumably the answer is basically the same for all knowledge work (and maybe all professions?). I think in this world the most likely background situation is that humans are not really useful for proving theorems, but nonetheless lots of basic and understandable questions remain open, including many that are open today (since I think these have basically unbounded difficulty). And lots of other questions of basic interest have been resolved. So human activities might include (1) trying to understand solutions, (2) trying to understand progress and obstructions to resolving open questions, (3) (non-rigorously) understanding mathematical phenomena. In all these activities, the purpose of the human is to serve as a locus of understanding. There's an obvious question as to why we would pay someone to do this. One plausible answer is that maybe we want to avoid complete disempowerment--at a minimum we might want people to understand what they can about what the AIs are doing--which requires development of human capital.