There are so many presumptions about what a pegged side chain to BTC would look like.

That is to presume the security of the SC would be equal or greater than the current BTC master chain. I highly doubt that will ever be the case.

If there was ever equal or greater security on a SC then why not have everyone just switch to the more secure chain?

To peg them and say they are "equal" and coins are transferrable between them makes little sense as one chain will always be more secure or have more infrastructure to support it.

Seems like a broken idea to me.