This makes me ask one question.
Is this content cut (or not put into) the game's initial release so the games can ship on time. And we pay for the content they had no time or were too slow to make by the release date. Or is this a really finished game with just more content?
Well, if not for DLC then the content would have been cut and then
maybe would make an appearance in the next game in the series. So it's a "finished" game. As someone who likes to tinker around with making games, I can understand when I've heard some developers say that a game is never truly finished, because there is always more you could add into the game or improve. It just gets to a point where you have to say, "this is it" and release it.
Edit: Oops. I totally forgot to post about the subject this thread is about.
Anyway, I'm definitely going to miss Break the Targets, but honestly, we don't really know for sure if it really is gone or not yet. There could still be more to Smash U than what we've seen. We haven't seen enough menus to know that Break the Targets is officially gone. I don't want go get anyone's hopes up, because it may very well be gone, I'm just saying, it's a little early to be etching an epitaph on a tombstone.
The best case scenario, I think, is if we can make our own Break the Targets stages in Stage Builder. Then missing the actual mode won't be of any concern, because we could make or remake any BtT stage.