Is that really what Sidechains are aiming for? If so I need to go back and reread their paper more carefully.
What I'm hearing you describe is that the token used by a Sidechain is not one iota different from any other Bitcoin, except perhaps that its latest output script includes some new weird feature we haven't seen yet. It's like my sending you 1.0 Bitcoin through a vanilla BTC transaction and you telling me that I now possess 10e8 Satoshis, which are different that Bitcoin but always redeemable for Bitcoin pegged at that rate.
If tokens being used by Sidechains are permanently pegged to Bitcoin, and we assume merged mining, I withdraw my objection to the concept, and would instead direct my criticism toward how the concept was presented and its seeming suggestion that Sidechain tokens might one day detach and rise in value while Bitcoins potentially fall to zero.
I would point out that you're not making a fair comparison to the cost of administering a fiat system. In the US, it includes the cost of the entire Federal Reserve apparatus dedicated to managing the supply of FRNs. Treasury's printing of them is only one small part. The Fed has buildings to maintain, support employees to pay, behind-the-scenes political processes that play out and who knows what changes hands there in exchange for appointments and other policy stances.
You're right that Bitcoin by nature includes a huge energy cost, but when you take the narrow view that it's simply being thrown away, it's both short-sighted that it ignores the theoretical possibility that its heat "waste" could be channeled into something economically desirable (e.g. steam to drive turbines), as well as not an even-handed to comparison to the true costs of its alternatives.
To be clear, I don't claim that non-Bitcoin cryptocurrencies do not have value. You're right that their de facto value is revealed through trading activity on exchanges. I view value as subjectively revealed by market prices, not through any sort of labor theory of value or other nonsense.
I'm arguing that non-Bitcoin CCs don't really add any marginal value to the CC ecosystem, and that it's inherently unstable for multiple CCs to coexist. Whatever rationalizations people are making to justify mining Litecoin and holding onto it (ignoring sunk costs of Scrypt ASICs) are inconsistent with reality.