I have referred to this situation as the ledger being distributed on multiple chains instead of sitting on a single one.
This notion has potentially positive implications for the preservation of the ledger : a single Blockchain is a single point of failure.
This probably isn't the point you were trying to make here.
You should realize that this single point of failure issue is compounded rather than being resolved by side chains.
Each SPV link creates an additional failure point in that model, each of them being only as secure as the weakest predecessor.
(If MC fails, SC
1 fails to the degree that SC
1 value is derived from MC and etc.)
I'm a fan of side chains. I love the potential it offers. I'm also a fan of Blockstream. These are heroes of the first order.
A hero ventures forth from the world of common day into a region of supernatural wonder.
Fabulous forces are there encountered and a decisive victory is won.
The hero comes back from this adventure with the power to bestow boons on his fellow man.
However to succeed, against some of the most powerful adversaries that we know of, the challenges are not small ones.
The victory is not pre-ordained. If we do not look for the pitfalls, and only look at the beautiful horizons, we will end up in the pit... with spikes through our necks.
Recognizing and examining the problems is part of what makes the adventure more likely to succeed. It also helps folks to see what this can do.
...
"The Ledger" in this context is a marketing term so I don't know if it refers to everything on any block chain anywhere, or the just Bitcoin block chain, or Bitcoin plus SCs (so long as the SPV is working), but then it becomes two ledgers if it stops working, or a chain dies...
It can mean whatever you want it to mean, but that just makes the term meaningless for anyone else.
Side chains do present a clear and present danger for Bitcoin. They also offer some extremely powerful advantages, but we can't hope to get any of those without dealing with the dangers. There are just too many pits, and we should recognize that not everyone wants to see either Bitcoin or Side Chains succeed.
Like Adrian-s and cypherdoc, I do not think we NEED side chains for Bitcoin to succeed, so I like a cautious approach. It is more important to do it right, than it is to do it.