First, a few links describing the term asymmetric warfare as it's generally used:
https://www.britannica.com/topic/asymmetrical-warfare
https://warontherocks.com/2015/06/bad-guys-know-what-works-asymmetric-warfare-and-the-third-offset/
Whoever would attack blockchains are engaged in asymmetric warfare. Attacking a blockchain is very expensive. Switching to or coding a new blockchain is very cheap. The attacker in this scenario will spend a ton of resources to win a battle that does them very little good to win. We just shift to the next battlefield, restart quickly and without taking nearly as much damage as they've sustained.
Another aspect of this thought applied to the blockchain world is how many different kinds of blockchains there are already. POW, POS, combinations, then others (at least proposals) such as the tangle and hashgraph. This means that the attackers would have to not just switch their mining power from blockchain to blockchain squishing them all, they'd have to mount an entirely different kind of attack many times.
It's not feasible. No organization or entity who wishes to attack and shut down this blockchain idea will be able to fight for nearly as long as we can asymmetrically defend ourselves.
