This is a ridiculous take. So if we are not able to develop a vaccine, despite their being an unprecedented amount of attention, work and money pumped in the efforts to develop one, and if there's no immunity, despite this not being necessarily likely, and if it turns out that the subsequent infections are significantly worse each time, which again is not necessarily likely, then we are looking at the extinction of the human race. Of course that's only if there aren't any advancements in treatments (again unprecedented amount attention, work etc. and so forth), general virology, education and the behaviour of the general public or any extreme measures that would surely be taken if there would be a world ending virus on the loose.
Sure, if all of those conditions are met then humanity might be wiped out, it's a possibility. But it's not likely now, is it? There's also a possibility that another completely unrelated and far deadlier virus might emerge or an asteroid could hit Earth or a gamma ray burst could happen or Yellowstone might erupt or one of the current dumb world leaders could trigger a nuclear war or any one of the other million doomsday scenarios might happen. They're all possible, but none of them are likely and unless your intention is to fear monger they have no real place in any reasonable argument, point or prediction.
It’s actually kind of fascinating. 
