They would have to commit suicide (the internet kill switch) or EMP which is likely not to be 100% effective. Of course that would mean no more Swift Network for the banks so back to pencil and paper accounting for them. Obviously they won't do this willingly.
What will happen is that for a time, hope that scaling can catch up in bitcoin and other cryptos so they can handle the load of Visa / MC. Lightning network is one hope. Until then, if people can just gradually spread out their investments into alts, that would ease the strain.
The alts are looking like the search engine wars with bitcoin emerging as the Yahoo! of finance. The Google of finance isn't here yet.
Maybe the google of crypto is here, it just isnt doing much yet?
It might be bitcoin if lightning network can solve the scaling problems.