If a sidechain is integrated into Bitcoin-qt or blockchain.info to the extent that you're not even aware of the sidechain being used, then I can see the scenario you're describing as being confusing and damaging to Bitcoin. But I think SC's will be implemented in a manner that it is clear when they are being used.
At least that is my expectation / hope.
I share the hope but not the expectation.
When speaking with someone about Bitcoin and they have only been involved since maybe 6 months or so (started after all the gox events), and they say that they have some bitcoin, and then during the discussion you learn that their bitcoin are in their Coinbase account, or their LocalBitcoin account....
Do you then tell them that they are wrong and do not have any bitcoin?
How likely are they to believe you if you tell them this?
There are a lot of these people.
Most people think that the dollars they have in their deposit account at bank are theirs and not the banks too, and won't be convinced otherwise without a lot of work.
...and that they own gold if they have some GLD.
That's a great way to put it.
It also means the problem already exists today. What is different between coinbase failing and a sidechain failing. I'd propose very little.
When MtGox failed, a lot of the press said Bitcoin was hacked even though we know that wasn't the case. A SC failing will be very similar.
It's not that it isn't a problem, it's that it isn't a
NEW problem.
Well... Bitcoin
was actually hacked (malliated transactions), but that wasn't what caused (most of) mtgox's problems, it was just used as convenient cover for the other problems. MK compounded the news confusion by blaming it and maybe bought a few more days, so MK should get the blame on that newsfusion more so than the news that reported it.
I don't know if it is a new problem or not, but the explanations and creation of understandings in the minds of the broader public may be made more difficult by the "one ledger only" notions. EmptyGox very clearly had its own ledger which at some point became irreconcilable with the Bitcoin block chain. If it were a side chain company that had monkeyed with its code to do these shenanigans, it would possibly been more noticeable (if it were being openly mined by folks that examine source changes), but if they managed to keep it hidden, or were privately mining (like Ripple?) it may also be more difficult to explain to the "one ledger only" believers how this is possible.
In the earlier example where scBTC
1 failed and scBTC
2 became an altcoin, at some point in this process there are very clearly (at least) two separate ledgers.
I would suggest that at the inception of each of the side chain's block chain, there and then emerges a new ledger, which are each potentially merge-able. Others might suggest some other time. Maybe at one of the many changes that resulted in the failure of scBTC
1, or maybe after it fails, or maybe never and the scBTC
2 altcoin and Bitcoin are still just one ledger. In that case, when the ledger of scBTC
1 failed, some might get confused and say the Bitcoin ledger failed. (Since they have been told that there is only one ledger, and something failed, so it must be that.)
It seems simpler and more sensible to me to consider each side chain its own potentially merge-able ledger. It is hard enough to try to explain how a ledger is merged with "something" when there is only one ledger in the first place, with what then is it merged?