I guess the point is that theoretically PoW could be attacked, but it would be vastly less expensive to attack PoS. In fact, the greater the attacker's credibility, the cheaper the attack would be.
Slight issue. With POS (at least in an implementation such as NXT), by the time you bought up enough of the coin supply to "attack" the network catastrophically, the only holder affected significantly by such an attack would be yourself

Put another way, it would be like buying somebody's house off them so you could burgle it

Not entirely true on two accounts: a credible attacker would be able to buy the coins cheaply from stakeholders based on his announcement that he intended to destroy the coin. Seeing that the attacker was credible and had a high chance of success, stakeholders would sell him their coins at a fraction of their "true" value. Thus the stakeholders would be harmed.
Such a bad actor would certainly have non-economic motives (at least with respect to crypto--perhaps he owns a bank or something which will benefit from destroying the "competition"), and in that case would not mind destroying the value of a currency which he holds 51% of (and which he acquired at dirt cheap prices from panicky sellers).
Your analogy is close, but slightly off. It would be like buying somebody's house and then bulldozing it. In fact, that happens all the time--for roads, malls, high rise buildings, parking lots, and the like.