Is security not an illusion with the current computer architectures? Sometimes it feels like we have to go back to architectures that separate code from data (Harvard) to really be secure.
Nah, you could still e.g. use a use after free to smash the return address or a vtable on a Harvard machine and execute a ROP chain. If it did speculative execution too it would probably be vulnerable to spectre. There's nothing special about Harvard machines when it comes to security.
Imho, to use a metaphor, not much different than the glass windows of a car protecting the car contents. And then of course, some owners just leave the windows down, other 'car windows' have special foils, multilayer, some even have bullet resistent qualities, but give it time and physical access and it's always a car compromised in the end.
Indeed. The NX bit (which differentiates between executable and writable data) roughly approximates Harvard Architecture-like differences between types of memory, and likewise it cannot prevent code reuse attacks.
