But for this to work it has to be tethered to the BTC chain...and there we'll have our Bitcoin world war II
In fact, I foresee considerable opposition to implement sidechain hooks in Bitcoin Core. But sidechains can add so much value to Bitcoin that the change is likely to be implemented. If not, Bitcoin might soon become obsolete and give way to an equivalent network that implements extensibility via sidechains.
I wonder what happens when the regulators realize that sidechains can put privacy and anonymity back ;-)
It won't be "considerable" this is likely to be all out war, think about the statements made by core devs with regard to alternate coins.....then they go and cook this up.... so they will get immense push back for that, then there is the personal interest vector, then there are those who will oppose a means of getting back @ them for this block size issue
All this before technical debates have even started...
What you're underestimating is the duration of the war. Sure, it will spread confusion, anger, defiance etc, but trends are so short these days, it'll probably blow over in a week or two. XT comes to irritate every now and again, but the enthusiasm doesn't last long.