Ruby 1.8 to 1.9 was very painful, I am not sure why it was more succesful than Python 2->3, I'm not sure it "deserved" to be or was any less painful on it's face. It easily could have been just as disastrous. So also informed by that; ruby hasn't done anything nearly as painful since.

But that applies to making it so old code does not work in the new version of the language. Nobody expects all new code to work in the old version of the language. Ruby adds new features including syntax that won't properly parse in old interpreters all the time. It's not clear to me why inline type definitions couldn't be such.

It was, and I was around during one of his discussions on that at RubyConf last year. It's a very valid concern and Matz is very sensitive to it. There are a lot of things he's joked about removing or changing but won't because of those reasons.

If you take a look at his keynote video he says quite a bit on this too.

And also the Ruby 1.8->1.9 experience.