Do you really think, professional miners would trash their expensive hardware, they can't use for anything else than mining, when the difficulty goes up? That would be pretty stupid from an economical POV.
no they don't of course. That's why the attack takes place at exact the time many miners close down due to being unprofitable anyways (on their own). But it says that in the OP aswell. Short: Anticipated natural decline of hash (due to being unprofitable) would be reinforced by the attacker.
The attacker uses an anticipated and predictable decline in hash (which would be happening without him doing anything) to his advantage.