Another option is to enable language features on a per-file basis. That way you could have your "ideal" syntax, and only have to do the work of updating those files where you need to use it.

The only language I can think of that does this extensively is Haskell (and some flavors of regular expressions). The main downside I have noticed is that the ability to do this allows for the language to add a bunch of complexity that would otherwise be prevented by the need to preserve backwards compatibility [1]; but, if the language writers can remain disciplined, it seems like an effective way to grow a language without the growing pains the article is discussing.

For adding keywords, this approach doesn't even add that much maintenance cost, as all the additional complexity is confined to the lexer.

[1] Or, as has happened several times with Haskell, add features that too broken to even be considered adding but for the fact they are always behind a flag.