IIRC it was just a personal preference of Graydon's back then. I'm also not sure it was intended to live forever, just something to try and nudge things in a particular direction.

Terseness doesn't inherently mean less readable.

I do think that that rule is probably not one that would be good permanently.

Probably for the same reason that most new language these days cannot bring themselves to just use "function" and instead have "fn", "fun", "func", etc. It's a headlong pursuit of conciseness for the sake of conciseness.