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.