Very unlikely it will be turned off, the miners could still keep mining other coins while mining bitcoin blocks even if there is no BTC reward. See "merged mining" https://blog.bitmex.com/the-growth-of-bitcoin-merge-mining/.
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.