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.
To fully comprehend whether this is a concern requires some terminological cleanup. I think there's a basic misguided focus in the term "sidechain." It's not the chain but the peg that matters.
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'Terminological cleanup'? How about 'basic critical thinking'?