The backslashes are not really relevant to the problem discussed in the post. The ambiguity between symbols and argument-taking macros exists just the same in LaTeX. Consider:

    $ f_\abs{x} $
    $ f_\pi{x} $

LaTeX just happens to do what the post calls "runtime parsing" because LaTeX doesn't really distinguish between different compiler stages at all. If you look at a macro, you can't know whether it will eat up the following braced argument.

In fact, not using backslashes for symbols can actually give an _edge_ with this problem because it would allow distinguishing `pi` and `#abs` (option E in the post).