That's only about the black death, and some specific reasons for why that helped, kind of, _some_ parts of Europe. First, it definitely didn't help everyone - Norway, for example, lost all economic power and went into the 400 year night, as it's called (it was under Denmark). And secondly, it's a single case. You can't create a general rule from that. It's vastly different to compare that case to when e.g. 95% of the population died out in certain places during the Spanish conquest.

And, again, take that "rule" to its logical conclusion: How many people will inhabit the Earth after a while, and under what conditions will they live?