What is the term for dystopic speculation as a debate tactic

To follow up on @Robusto's comment, any of the following terms would work. From Wikipedia:

In logic, reductio ad absurdum (Latin for '"reduction to absurdity"'), also known as argumentum ad absurdum (Latin for "argument to absurdity"), apagogical arguments, negation introduction or the appeal to extremes, is the form of argument that attempts to establish a claim by showing that the opposite scenario would lead to absurdity or contradiction. It can be used to disprove a statement by showing that it would inevitably lead to a ridiculous, absurd, or impractical conclusion, or to prove a statement by showing that if it were false, then the result would be absurd or impossible. Traced back to classical Greek philosophy in Aristotle's Prior Analytics (Greek: ἡ εἰς τὸ ἀδύνατον ἀπόδειξις, lit. "demonstration to the impossible", this technique has been used throughout history in both formal mathematical and philosophical reasoning, as well as in debate. [Bold from Wikipedia, italics mine]

I see nothing of significance in both debaters attacking one another using the same tactic, other than that their respective arguments were susceptible to the same kind of counterargument.

Of the above terms, argumentum ad absurdum may be the most natural/literal descriptor of the term you're seeking.