It depends on how it's implemented.
On the contrary. It depends on the principle - the implementation is irrelevant. If you read the
sidechains whitepaper, the principle of 2-way pegging underlies the whole thing. This is not going to change with 'implementation'.
If the Blockstream design was as you say, a direct modification of the properties of Bitcoin and it's monetary units, then I would understand and agree with your criticisms
direct modification is a relative term in this case. It is not a 'direct modification' in a technical sense but it is in a commercial sense since the bitcoin is removed from circulation and directly substituted for the sidechained-alt. So it amounts to a "direct modification" as far as the market is concerned (thats the whole point of sidechains).
I don't expect accomplished computer scientists to come up with such glaring design faults.
"accomplished computer scientists" are not necessarily "accomplished monetary analysts" or even "accomplished systems analysts". It's true that the people who design good software systems are usually programming professionals, but the people who specify the requirements for such are usually not.
In this case, an implicit assumption has been made which is economic - i.e. that the world economy cannot sustain more than one cryptocurrency and that "network effects" will tend towards the adoption and consolidation of this currency. But then the theory immediately contradicts itself by acknowledging the need for diversification and hence the emergence of sidechains.
You only need to take one look at the
range of monetary like stocks traded on any global exchange to see that markets favour diversification of genuinely independent assets rather than diversification of 'presentation' within a particular asset, so the commercial assumption that underlies the concept of sidechains is flawed IMO, however well designed it is technically.
This analysis is further endorsed by examining the properties of "good money" which are well understood and considering the impact of a concept like sidechains on these. Think it through - you basically have a set of pegged altcoins that have varying levels of demand, all pulling in different directions on a common base currency. The effect of this is to cause price and liquidity volatility in commercial sectors where it didn't originate. What will the market do in that case ? - it will simply use an altcoin that's native to that sector in preference to the sidechained one.
That is one of the effects of adversely impacting on "fungibility" but there are others (think Greece & the Euro).