Speed is not the only benefit of RAM. The internal data structures of most programs are gnarly messes of intertwined pointers, fragile, easily corrupted, hard t

Speed is not the only benefit of RAM. The internal data structures of most programs are gnarly messes of intertwined pointers, fragile, easily corrupted, hard to upgrade in place, almost impossible to fix when they break. The division between RAM and disk forces everything to be serialized at intervals to clean storage formats that can be reread for a fresh start. As a practical matter, that's a big enough benefit to justify enforcing the division at an API level even if disk were as fast as RAM.