I worked for a few companies that built and provided the equipment and software used in the Enterprise Internet Provider business side. I worked in the business for 12 years. In other words, not the companies that provide Internet service to us pions but the companies that actually owned and ran their segment of the actual Internet on a statewide, US wide and worldwide basis. Some of them are now one in the same. I worked on the development side testing and supporting Global Internet Applications and Internet Security Applications that ran on the routers, switches and servers. Later on I tested DSL Hardware and then eventually went on to testing and development of Storage Area Networking Hardware in the Enterprise Business side. In other words what we now call the cloud. I mentioned I supported the software I worked on. Which meant I was on call (with pager) 24 hours a day and my on call shift would sometimes extend up to 2 weeks at a time. If things went down I was called as a third level support member. In other words, after the level 1 and 2 support engineers failed to solve the issue. If called I would immediately go into the office, and if it could not be fixed in the office I would travel with the developers to the site. The first hardware company I worked for guaranteed we would be on site within 24 hours if the issue with our software could not be fixed within 4 hours.
In the first 5 years I was in the Enterprise IP business side I was only called on 3 times. And one time I had to fly out to RTP North Carolina from SoCal. The Internet is mirrored and all equipment is duplicated at different sites. So if one state/segment went down the mirror picked up the load. There is always enough equipment dedicated to keeping the Internet running under almost all segment down conditions. When something does go down it usually means somewhere there was a single point of failure that was not taken into account for. So if the load increases to the point that one link can't handle the second will take up the slack. But before that happens whoever owns that segment is already installing more equipment in order to be able to handle all traffic with a single link in case one goes down. There are always plans in effect to keep things running. Believe me, the companies that own this equipment already know how much bandwidth they are going to need 5 years from now and are already planning for it.
Also, all data centers have backup generators if power goes out. Because, the one point of failure we have here in the US is the electrical grid. If the Internet goes down, most communications will cease. We won't die. HAM radios will rule. Satellite could be used by those (US Govt) who have their own link that is not dependent on the commercial providers. So how likely is it that the Internet can go down? Here in Hawaii quite possible. I was called into the office once when the whole south half of Florida went down because the hardware they were using crashed and their mirror was never tested. Our software was blamed but in the end I didn't have to go out there because it was a hardware/IOS issue. Anyway, we will be disrupted and pissed off. And we will be playing a lot of solitaire. But the end of the world? Doubt it. I would be more worried about our electrical grid and not having electricity.