This is absolutely not true. Today it is hard to find even a simple [ARM] micro without memory protection unit. It is typically not very capable, but good enough for most applications.
Higher-end MPUs and CPUs have very good memory protection mechanism. Go ahead and try to crash something other than your own application under any modern OS.
It is like saying C is not secure, lets all use LISP. There is a natural selection, and nobody wants LISP, however good it is at calculating a factorial.
Same goes for MCUs. What we have is probably not the best, but most definitely good enough. And any proposal that claims to solve all the problems at once is bound to fail.
This has been proven wrong many times and it is also not very relevant in an age where hardware comes backdoored from the factory. Memory protection is at best a hack.
Turing and VonNeumann could have told you even back then the properties of a secure cryptographic or computational system. These properties are fundamental and if not observed no subsequent hack can remedy them.
1 -Cyphertext key and cleartext should never ever be stored in the same place. 2 - The only place these three come together must be in the cryptographic engine, 3 - which should have no persistent memory and should not be Turing complete. It should be a finite state machine with only enough states to let it do its job.
From this you can infer that no computer is fit for cryptography or security work because it is Turing-complete and also has memory.
You can extend this to imply that storing code and data on a Turing-complete machine is also fundamentally insecure. A Turing-complete system is incapable of keeping secrets from itself. You can't change this fact by merely writing more code for the Turing machine to compute.
The Berkeley architecture mitigates that to an extent but the mere fact that it is not the dominant architecture on the market today indicates that there are other non-scientific factors at work.
This also means that even with Berkeley architecture you still can not store the above three elements on the same machine.
Btw has anyone counted how many separate Turing-complete systems are there in a computer?