It will only break existing programs if they start introducing Type! into existing APIs. I understand that introducing ! types in existing Java libraries would be nice, but I much prefer the ! mechanism to be more "correct". I agree that it will be a big improvement either way.

It seems we need some annotation akin to @Deprecated(forRemoval = true). Something like @NotNull(willBeEnforced = true) that would only emit a warning but informs the user that soon there will be a breaking change related to nullability.