Perhaps cypher's concern is that everyone moves their BTC over to the sidechain, then the sidechain fails in such a way that it's not possible to reconvert them to BTC, and the whole Bitcoin ledger is destroyed.
Probably not else he would not have studiously ignored my question about how that differs from one simply losing a private key in Bicoin which has happened a zillion times already. Bitcoin designers saw no reason to address this 'weakness' because, in simple terms, it is no big deal.
This is a little different: It's like people switching over to a "better" alter-ego of Bitcoin only to realize they made a grave error and all their money is lost, destroying confidence in Bitcoin and ruining the ledger that has been built up over the all these years. We'd have to start from scratch, which isn't the end of the world but it's a lot worse that just losing some private keys. I tend to think this is a bit unlikely, though, simply because I can't see "everyone moves their BTC over to the sidechain" until it is absolutely solidly established that it is no more dangerous than Bitcoin is. That could take years, if it ever happens.
To Bitcoin there would be basically zero difference (between a sidechain crashing and burning and an individual losing their private keys.)
I don't know about you-all, but my interest in sidechains is nearly 100% about preserving Bitcoin and that is where my focus is. Sidechains allow Bitcoin to freeze in it's current fairly reliable, predictable, and defensible state while not hampering the ability of it to server as the 'everything to everyone' value foundation. That is the big appeal to me.
I'm not worried that a failure in a given sidechain will rub off on Bitcoin. If anything I think that people will see that with sidechains, failures are not catastrophic to Bitcoin proper. I'll bet that there are a lot of people who are, like me, deeply concerned that Bitcoin itself is a single point of failure and efforts to grow it into the single currency for the world's masses are almost certain to trigger one failure or another and more likely than not a catastrophic one.
I'm looking forward to the new worlds that will open up as various sidechain efforts occur, and I hope to eventually use some of them for things I cannot do with Bitcoin right now, but that's pretty secondary.