> Lisp purists will argue M-expressions are unnecessary and S-expressions are all you need.

I would argue that S-Expressions turned out to be extremely practical for developing Lisp software. A layer with different syntax makes it more complex to use.

> Maybe if the M-expression idea had been pursued more seriously, Lisp might be more popular today.

Maybe, maybe not. People have discussed it endlessly, but there is no conclusion.

Currently some people in the Racket community try to make Racket more "popular" (widely used, ...) by providing a new syntax.