> is that the original source isn't solely responsible for keeping the content online and the link working
No, the original source is still the one solely responsible, unless there's some agreement between parties to cache content for them.
Anyone can help, yes, but it's not their responsibility to keep content online unless there's some agreement between the original content source and other nodes, which is of course not the case by default.
Content-addressing doesn't alleviate the problem 100%, since content can still fall off the network - but it improves the structure of the network in a way that makes it tremendously easier to keep content around. You don't need control over the original domain name to help in distributing a certain piece of content.
No, the original source is still the one solely responsible, unless there's some agreement between parties to cache content for them.
Anyone can help, yes, but it's not their responsibility to keep content online unless there's some agreement between the original content source and other nodes, which is of course not the case by default.