> Yet nobody has ever been able to justify why it's necessary to dispense with the entirety of C and C++ to achieve memory safety.

As soon as you make a backwards incompatible change, you've already made a new language, effectively. Because:

> For example, you could create a fork of C++ that merely compile-errors when you use a pointer and not a reference unless you enclose it in some `unsafe` block that you invented

This means that effectively all C++ code would fail to compile under this implementation.

> One can simply start with C++ augmented so that everything is const by default; that is the ideal progress to me.

This is nowhere near enough to achieve the goal. Sure, it's a thing someone could do. But it's kind of irrelevant.