If this thing happen in the future, I ma pretty sure that the Altcoins or Bitcoin will surely be dead instantly, that's the reality in which we need to accept it. due to nowadays, internet wherever we go it is always part of the business in the entire world now. Especially, in terms of communication in different countries and others as well.
Hope you are aware that you will be able to move Bitcoin offline and you only need internet to validate the transaction in the block, first and foremost there wont be a situation where there wont be internet for an extended period of time and if you want to make a transaction during internet outage you can do that and once the internet is online it will be validated.