It's not a zero-sum game: value can and will be created and destroyed. If a crash of tether causes a distrust in the whole crypto space (which is not a given at all, granted), and people rush out of the whole space, all of it will crash.

Would a crash of tether be a source of concern for the value of Bitcoin? If bitfinexed hypothesis is correct, that is to say that Tether is printed out of thin air and used to buy Bitcoin thus artificially inflating its value, then yes, absolutely: a crash of Tether would cause massive distrust in the entire crypto space...