Everything you said applies to stable sort. The reason
std::get_temporary_buffer exists is because it was believed that operating systems would have a way of allocating memory based on memory pressure, such that if you asked for 50MB and that request would end up being slow, but 20MB could be returned cheaply, then you'd get 20MB of memory instead.
Anyways, that never happened, but the
stable_sort implementation is required to work whether it gets a temporary buffer or not and there's no real reason that the sort implementation in either standard or core can't use that strategy.