Bitcoin the currency will never leave Bitcoin the Blockchain, even with sidechains. Sidechains just gives you more potential uses for your Bitcoins.... locking them gains you tokens that have added functionality.
Side chains create a place to experiment with new functionality, and also create people who have an financial incentive to make sure that functionality never makes it into the main chain.
Do you where else it's possible to test new features?
Testnet.
Where is it possible to test features too invasive for even testnet?
The btcd team has produced a great feature called
simnet where you can easily bootstrap a small mining network that tests modifications of the Bitcoin protocol.
All the tools needed to develop and test safe upgrades to the protocol already exist.
If the Bitcoin Core team doesn't have enough software engineering and project management resources to push the protocol forward now, employing half of them at Blockstream and adding new opcodes to Bitcoin isn't going to magically fix anything.
You know, the Blockstream idea must have been going on for at least a year.
Had anyone stopped to think that maybe no innovations have gotten done because the core devs involved with Blockstream have wanted to strengthen their case?
The first time I heard about this idea (before it was called sidechains) was a conference where I met Adam Back. Pretty sure it was the first Inside Bitcoins conference in Las Vegas, if not then it would have been the 2013 San Jose conference.