[uncletammy]

This is a good point that the authors don't touch on. There are many coins secured by sha256 and I'm sure many more will come along before BTC finishes it's release schedule.

Switching to any one of them is likely to provide more revenue than just attacking the network for a little extra transaction fee money.

This is one of those times it's really useful to make a distinction between bitcoin the currency and bitcoin the protocol. If it was the authors intention to describe a future problem on the BTC network then they messed up by not addressing the rest of the sha256 mining ecosystem. However, if it was their intention to describe a problem with the protocol itself then it kind of makes sense to not touch on miner's alternatives.

[tromp]

All other sha256 coins combined only account for less than 2% of bitcoin's hashrate, so this doesn't really change the arguments.

[bouncycastle]

No, this is not what we are discussing. We are discussing using bitcoin's hashrate to mine other coins. No modification to bitcoin is needed, the other coins simply need to be aware of bitcoin and be able to verify bitcoin's blocks. Other coins can have an auxillary mining algorithm too, or even merged mined with other coins besides BTC, doesn't have to be sha256.