The problem is that the bad block cannot exist in the first place. You cannot spend from an output that you do not own. Segwit does not change this behavior.
before segwit is released someone can make a segwit tx. where the witness data is not part of the transaction.
a better analogy than anyone can spend is that any signature data is treated as just a 'message'. that is overlooked and not checked or done anything to.
old clients will just see it as a funky tx.
again segwit tx is not a op-true so please look away from the anyonecanspend analogy and concentrate on op_0. and only op_0.. they are different things entirely.
so knowing old clients are backward compatible to blindly look passed segwits. it proves that old clients would not reject or orphan a block containing a segwit. otherwise segwit is not backward compatible. and blocks would get rejected all the time by old clients.(if the opposite to reality could occur)
its just basic logic/common sense.
old clients wont reject a segwit transaction
and nothing in the future when the real segwit it activated changes for old clients.. remember old clients wont upgrade so nothing changes for them. what they look blindly passed now they will look blindly passed in the future.