It seems that the institutional banking industry is taking note of cryptocurrencies as a potential threat. And so there is an incentive there by some players to

It seems that the institutional banking industry is taking note of cryptocurrencies as a potential threat. And so there is an incentive there by some players to want to destroy coins without direct reward. But even this scenario is I think somewhat guarded against. Attacks on networks are completely visible to everybody. In light of an attack, a network could be forked, illicit transactions rolled back, a new proof method adopted if necessary, but otherwise keep the entire ledger intact. And indeed if/when this happens, there will likely be numerous forks that try to become the new standard. And this could actually end up being, in net effect, profitable for holders. Think about Bitcoin vs Bitcoin Cash. Bitcoin Cash was a somewhat controversial fork that some feared would split Bitcoin to unknown effect, but in the end it's only effect was giving all bitcoin holders an additional $754/coin (price as of today). In a time when a network was compromised, there would be a large market incentive to takeup the forks which would send their prices skyrocketing. Coins may end up being like the hydra. You slice off one head only for 10 to take its place. In a way this seems to be true of decentralized systems in general. Each time e.g. a major torrent site is taken offline, a dozen more come to try to fill that vacuum that's been created.