It is in the nature of bash that you can go a decade without ever figuring something out only to have your life changed by a random comment on the internet.

So thank you. That changes everything!

This is amazing! I don't think I'll use it for help (up-arrow, ctrl+w is fewer keystrokes), but I can think of many other situations where I want to substitute one word of a very long command and didn't realize it could be this easy

I think

  ^help^

does the same thing. The bash manual says ^string1^string2^ is equivalent to

  !!:s/string1/string2/

where / can be any delimiter, and the "final delimiter is optional if it is the last character of the event line." So, not so surprising that the third ^ is optional too.

TIL: In either of these forms, if string1 is empty, it is set to the last string1, or, "if no previous history substitutions took place, the last string in a !?string[?] search." E.g.

  $ echo foofoo
  foofoo
  $ ^foo^
  echo foo
  foo
  $ ^^bar
  echo bar
  bar