It depends the expectations, aka how those 2 hypothetical persons are used to communicate together. Any change in said habits may trigger questions but that is still very subjective and, to me at least, it also seems to be a little bit too emotional in that theoretical example you briefly described.
Imho, a better reaction would be
- to wait (its not a matter of life an death, right? And if it was one better be contacting a doctor or the police)
- to ask the other if there is anything wrong going on and, if so, if there is anything one could do to help them. Instead of instantly jumping to any conclusion regarding their real secret motivations for not immediately answering a message... that more often than not is probably not that important to begin with.
If that helps you worry a little less: I may spend days before replying any message. It may be weeks or more than that, before I reply some emails. And btw, that’s true for those odd people that, somehow are still willing to exchange with me despite me apparently being so rude with them: they’re treating me as rudely as I am. Save that we don’t call that being rude, we call that being ok with not being the center of the other's life because, well, we are not ;)
Also, it’s ok to not stay 'friends’ forever. Things and people change and so will their interests. We all can change.
Edit: rephrasing