Comparing a successful fork to a runaway sidechain.
A chain fork, while painful, will succeed only if the fork is better than the original. A bitcoiner need only do nothing to join the fork.
A sidechain will either be less valuable and therefore be very small, or more valuable and therefore run away. A bitcoiner might be left behind, unless he converts to the sidechain in time.
I prefer a chain fork to a runaway sidechain.
Yes, chain fork = (Peter R's) spin off, though the latter carries a connotation of being done in a somewhat less chaotic fashion. As I said before, spin offs make a lot more sense than side chains as a way to (potentially) upgrade bitcoin. Even Adam's one-way pegs are better than side chains, but spin offs are better than one way pegs.
But Blockstream's two-way pegged side chains don't runaway from your BTC value. Thus they are best, because they are not all-or-nothing choices, you can go back and forth, and your BTC value is protected.
I consider the entire scheme unstable and unsound, and quite plausibly will never even be implemented (in other than federated form, at which point it instead becomes uninteresting).
Because of the vulnerability of the network to 50% attacks? But what if a side chain isn't vulnerable to 50% attacks.
In that case, assuming other more serious compromises aren't made, then your system is simply far superior to Bitcoin. You should replace it and not carry the Bitcoin system around as unnecessary and uncompetitive overhead.
If you can carry those BTC units on your coin's network orthogonally to your coin's units, then you get the benefits of leveraging Bitcoin's existing network efforts while ramping up your own coin.
Transition isn't instantaneous. Many may prefer to transfer BTC value and then later after convinced, they convert BTC to your coin.
Any other reasons you think it is unstable and unsound?
Economically I don't believe that two different assets can be successfully pegged to the same price, especially not in a decentralized manner (though doing it in a centralized manner also likely fails to a version of the calculation problem). If you extrapolate from this premise, it is clear that various failure modes are inevitable, some quite catastrophic. But possibly people are mostly smart enough to stay clear of the whole thing in which case the failure mode is non-catestrophic (a whimper not a bang).
Well long-term, it impossible to guarantee that the protocol of the pegged side chain will continue to warrant that it is not creating new BTC units, thus long-term there is no way the peg will be respected on the side of the side chain. The Bitcoin Core block chain wouldn't care because money supply there is not subject to change by what a side coin does. (ditto vice versa)
So yes the BTC value in the pegged side chain can not be warranted long-term, but that is not the targeted purpose. It is a short-term (couple of years or so) test of flow from Core into a new network. If the flow is sufficient, then it succeeds and eventually diverges from Bitcoin Core long-term. Short-term is enough to meet investor expectations.
I think you are making a catastrophic mistake to ignore them. I hope your view doesn't represent Monero's position?
Why is federated uninteresting? The masses don't give a hoot about theoretical decentralization (otherwise they wouldn't buy Bitcoin nor Monero nor any other existing cryptocoin in first place because none are theoretical sound decentralization).
Whose leg are we pulling here.
I don't really care what the masses buy, except in so far as I can successfully front run them.
Non-decentralized systems are uninteresting because there are many well known ways to implement them that don't have the cost and performance compromises nor the user-unfriendliness of blockchains.
But if that semi-centralized federalism is a pathway to a superior decentralized network, then that centralization gets discarded...
Careful the myopia...