I hugely disagree with the belief that sidechains will decrease the value of Bitcoin. Having more uses for the Bitcoin blockchain will almost certainly increase its value, rather than decreasing it.
Do you believe that Bitcoin the currency is separate from Bitcoin the distributed ledger?
If you do then yes Bitcoin the ledger can be used without Bitcoin the currency. (Make the protocol change)
Finding uses for Bitcoin the blockchain that are separate from the currency are a threat to Bitcoin the currency.
If you believe Bitcoin the currency is Bitcoin the ledger then separating them threaten Bitcoin.
I see all the value that SC enable, I also see some situations where the SC is more valuable than Bitcoin and in those situations bitcoins value leaks into the SC it's not that Bitcoin is beet by competition it's the SC that has enabled a mechanism that exploited Bitcoin.
Cypher has given examples and there are others too.
1:1 pegs are a dream. They will only exist within a trusted and controlled system.