The problem that me, and seemingly a couple others, have with Cypherdoc's concerns is that most of them boil down to the creation of altcoins, something that sidechains did not introduce and do little to enable. I hope you have come to understand this part because the accusation that you make of SC proponents being "unreasonable" can equally be applied to most, if not all, of the SC opponents in this thread.
You seem to be a reasonable person who has a great ability to distill his ideas in thoughtful and concise manners. Maybe you want to share with us notable weaknesses of the 2wp and what considerable risks you can foresee?
I don't know that there are any SC opponents in this thread.
I also don't claim there are any weaknesses in the SPV mechanism, and find it weird that you would see that anywhere in anything I wrote.
I like SC's, I think it works and does pretty much what it says it does is the blockstream white paper.
There are however risks, some of which have been discussed here already. Most of the more interesting risks fall into two main categories.
1) Economic risks (changes to miner incentives, centralization issues, and practical concerns, ZB's is one of these)
2) Confidence risks (both "cons" in the scam sense, and misplaced confidence in the mechanism as a panacea, or else misplaced confidence in any particular SC - what you call the altcoin risk, and loss of confidence in Bitcoin generally due to its new ease of change).
As mentioned upthread, the SC mods present a new upgrade path. Introducing upgrade paths is something to do carefully, and I like that there is this discussion to work through some of the more obvious issues.
There are all sorts of dystopian varieties of these two categories.
Some may imagine a distant future where with a click of a button (or it is done automatically) where one upgrades their bitcoins from Bitcoin 17.4 to 17.5 moving to yet another new chain, and something goes horribly wrong and they find themselves on Central Banker Inflation Coin of something.
Or what is more likely historically, they do this willfully because the news is telling them that it is necessary to do it for their survival in the new important war of the day.
Edit: there is a 3rd category of risk, technical risks (of which SPV brokenness would be an example, but these haven't been much discussed here).
Now these are reasonable, sensible remarks that do not rely on hyperbole scenarios and generally address the situation in a non-biased manner. Thank you.
While there are some issues presented that I don't see as a problem, some definitely merit attention. Of course, due diligence on the part of the user is always one of the main concern but while sidechains might introduce new forms of scams, I will continue to say that fools, no matter the mechanism, will find a way to part with their money.