PoW is economically broken because the margins tend towards zero? Seriously?
Yes. It means that to attack Bitcoin one must only make the reward higher to do so. And if the reward for supporting Bitcoin tends toward zero the cost of doing so is fairly minimal.
This in the best of worlds when the valuations are fairly steady (or their derivatives are.) In the real world we can expect a fair amount of sha256 being idle for significant amounts of time due to mining being a money loser. With luck it would shift to supporting the next scam-coin but sooner or later a sha256 scam-coin will realize that their best chance is to nuke Bitcoin and try to siphen off some of the userbase. In fact here's an idea for scam-coin designers that I just thought of: 'merge-demining'. That is, mine the scam-coin and fuck with Bitcoin at the same time.
In reality, as I mentioned, someone wishing to attack Bitcoin by better rewarding the failing miners will probably find themselves up against some money who wishes to exploit other aspects of the Bitcoin network which they alone can monetize. That's the same thing as Bitcoin dieing to me, but probably not to everyone. Many sheep would probably welcome it. Probably they'll get 'cash back' for using Bitcoin...just need to authenticate through the right framework...
Sidechains base their value on (being) Bitcoin so they have every reason to keep it healthy. They actually need no real POW (or other such device) because the backing role is taken by Bitcoin and the peg...as long as Bitcoin remains solid. They do need to interface with Bitcoin and that is done through mining which is needed to exercise their peg (maybe?) This is how I visualize sidechains at least, but I'm not the designer of them. I do see such a thing as the best hope for Bitcoin in a form I appreciate though.