Hippowdon can only take Close Combat so many times. Not only that, Terrakion would force its "counter" in, which is all it needs to use Swords Dance and be unstoppable. Not only that, switching into a Close Combat will likely take away over half of Hippowdon's health, guaranteed after Spikes/Stealth Rock. Otherwise, you're absolutely right. With a Curse set with Slack Off, that could be a pretty effective way to handle Terrakion.
Hippowdon has access to slack off for recovery, and only takes more than 50% damage from a choice band adamant close combat or at +2...the turn used to boost is a safe switch for hippowdon and jolly terrakion never does more than 45% (based on plenty of experience) ...entry hazards are something that can also be taken care of with appropriate team support...idk what you mean by forcing acounter in, but last time i checked, it doesn't get u-turn so whatever you mean, it could be abused by both sides...hippowdon could heal up while terrakion switches out, while it will ko terrakion if it chooses to stay in...it's all mind games my dear watson