As we've seen, all a 51% attack does is to produce a fork. In cases where the attack does not have majority support, the 49% can just continue to mine on the original chain, exchanges can continue to trade on the original chain, and the attacker will have received block rewards in a token that is worthless, the attacker will have wasted a lot of resources on non-rewarded PoW, and the attack will not have accomplished anything.
The reason Satoshi Nakamoto highlighted the 51% attack is probably because it is a trend for a whitepaper to have a bit of self-critique. Now 7 years later, we've seen real world examples of these attacks, and have a better understanding of them, and can see that the risk has been exaggerated as miners are only a minor part of the security model of blockchains.
As long as there are enough miners to prove that the blockchain remains intact, 51% attack "spin-off chains" would not really do any damage.
TL;DR: The PoW consensus is ultimately meant to prove integrity of the blockchain, not to prevent spin-off chains.