> I think of address space context switching overhead as the performance price we pay for not being able to run all our programs in a single address space, which we could safely do if we knew all the programs were emitted by a trusted compiler that disallows unsafe memory access. Imagine if system calls were just ordinary functions that can be called with no more than the normal function call overhead?

That's pretty much how the Amiga worked, and that level of technology achievement is still unsurpassed today.