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.
This is incorrect.
Sidechains provide a decentralized mechanism to implement new features, not simply test. This is the entire spirit of Bitcoin's decentralized & open design.
On the Testnet you can only test new features, not implement globally and drive adoption. The reason new features have stalled is Bitcoin has grown too big to easily change things. This a tremendous risk in the long run.
Today new features can only be implemented through centralized players in a highly political environment through global agreement to change at once, which is too hard. This both makes it next to impossible to implement new features and there is a risk of bad actors implementing negative features.
For example, let's say you want to add Zerocoin functionality. How is this even remotely possible today? The hurdles are so high that it is next to impossible. Also, assume you did do it and somehow got everyone to agree to the changes, but then you find out later the implementation was a little off and needed adjustment, again the hurdle is too high to make these corrections.
On the other hand, with sidechains
any one of us could easily create and fully deploy zerocoin functionality. You could make and launch version A and I could make and launch version B, and the best version would gain global adoption. That is infinitely better than the situation today.
This is competition, this is bitcoin, this is what sidechains offer,
decentralized implementation of new features. Instead of focusing on the beauty of this, 3 people on this thread are falsely beating a "sidecoins will cause inflation" horse that is FUD and simply not true.