Depends on how you interpret "mistake." I don't think that improving the language is necessarily a mistake, but the way they did it is a mistake. It should have been an absolute requirement that, one way or t'other, a user could employ 2.x libraries transparently. No change should have been allowed until and unless a way was found to make it not impact backward ecosystem compatibility. This is a law of nature that was ignored and 3.x is paying the price that anyone could have predicted (and most did.)

Not ensuring library compatibility was lazy, egotistical arrogance and will ultimately relegate 3.x to being a historical footnote.