> it’s more expensive to do “the right thing” (because if you’ve properly checked you’re paying the check twice and praying the compiler can remove the redundant check)

Eh that's hardly a solid complaint here. Since these are header-defined templates, it's all inlined. And any compiler will absolutely remove those redundant checks as a result. And an unchecked .value() would still be available if you really did object to that "overhead", so it doesn't violate any C++ principals here.

I do kinda think that operator-> & operator* as a result did the assertion, and value() bypassed it. But this would also be the opposite behavior of other std:: classes, like std::vector (operator[] being unchecked vs. at() being checked).