When I added __ms_hook_prologue__ to gcc a long time ago we did not anticipate a high number of functions to need it. In 32 bit x86 code it adds minor overhead - there is a 2 byte nop at the start of every function that uses it. Having it on all functions was considered unnecessary and wasteful. On x64 this nop is 8 bytes long.

At the time I worked on this Windows only made regular exported functions hotpatchable (what we have in our .spec files), but not e.g. COM methods. But Windows applications hook COM methods too, we wanted to be able to make static functions hotpatchable. On Windows this is a compiler option: https://docs.microsoft.com/en-us/cpp/build/reference/hotpatch-create-hotpatchable-image . Doing the same thing would not work for us because we still have to make sure that e.g. IDirect3DDevice9::Present does not contain code that Steam et all cannot handle.