Never has it been said anywhere that the link between the blockchain and BTC should be broken.
What Blockstream, and Adam Hill, are proposing is to allow the BTC to move between subchains and not be restricted to Bitcoin's blockchain features.
In the end, the BTC locked on a sidechain is still accounted for on the mainchain, as rocks has discussed.
I don't think you understand what I've beet talking about and why SideChains are interesting to financial asset management companies.
Bitcoin the private key has no value, securing it in a SC doesn't secure value stored in the Bitcoin blockchain, (just like securing my keys on an offline computer can't secure the value in the blockchain) only now it could be allowed to freely move into the SideChain. This is new this is a big change, this changes how I see Bitcoin, there are pros and cons, I'm not interested in understand the pros I have a lifetime to explore those I want to understand the cons.
And your not helping.