I disagree with Evan on this question and many others, but I just don't understand your criticism here. That seems like exactly a language designer's place? Someone needs to decide how the language is tokenized, what counts as a valid variable name, and so on. Very few languages have the answer "everything not explicitly used for something else is okay in variable names". (And Elm does use the ' character for other things, the rule which would allow ' would allow many many other things.)
Maybe you mean he should have made a decision and stuck with it? But that would severely limit exploration.
Is your hypothetical that Bjarne Stroustroup tries to do that now, when C++ is 35 years old and ISO certified? That doesn't seem a close match with Elm, which is less than nine years old and not yet on version 1.