They sure could, with the remaining 30% of the hash power. Thing is, continuously forking isn't a recipe for a store of value or currency or anything else.
You are right that wouldn't make for a good store of value. But Bitcoin actually does not get attacked "continuously" so that is not an applicable criticism. It is conceivable that such a thing could happen but we can expect it to happen approximately never given the incentives in place.
Don't they? Then what does? I just don't see what the benefit of conducting such an attack would be.
Coinholders would practically not be impacted: they could just take their existing keys and bring them into the hard-forked software and continue like normal. Mining might be disrupted for a short time, but is that so harmful to the enemy to be worth spending millions in electricity?