To be fair, the MM primitives that we have today are the same we had in 1979, while the capabilities of the hardware evolved, and the actual needs evolved, too.

To be fair, the MM primitives that we have today are the same we had in 1979, while the capabilities of the hardware evolved, and the actual needs evolved, too. For example, caching. It's essentially impossible to do caching at the application layer right unless your cache is also persistent with no durability or consistency needs whatsoever and can be mmaped (and you don't mind total I/O trashage). The persistence part also implies that you must be able to store the entire cache somewhere. What an application would want here is a mechanism to allocate cache memory that the kernel can throw away, if need be, and notify the application of that, so that the app can adjust. That's not impossible to do, it's just tricky - and beyond POSIX.