Why is that? It'd only be bad if it did not eventually stop with a stable non-zero level of population, preferably one above just a few tens of millions.
An aging population will cause major problems - just look at Japan's demographic time bomb. Sure, the problem will eventually go away, but below replacement birthrates are so troubling that even China has changed it's one child policy.
I'd have no problem with the earth's population stabilizing some time in the future, but see no need for the sort of massive reductions some here advocate.