What I mean is, most compilers won't generate a function call or an additional indirect memory access for *(111) = 222;, unlike your spec-compliant alternatives.

This is, IMO, the problem. Sometimes we just want a more ergonomic way than inline assembly in a hot loop (and also we'd like to to pretend its portable). The spec does not have an answer to this.

So, unless all C compilers in the world actively sabotage this usage (like how Go did to map traversal orders), your spec saying this is "implementation defined" have no impact at all. All C compilers are bound to implement the same intuitive but completely-not-enforced-by-spec behavior. Because if they don't, people just use something else.