Stung by Nettle

I've been reading David Nettle's 'Linguistic Variation'. It brings up a very concerning issue:
A common ancestor to humans lived about 140,000 years ago.
Assuming a generation time of 25 years, that's 5,600 generations ago.
There are about 6,500 mutually unintelligible languages in the world.

At first glance, this looks like there is at least one new language invented every generation! However, once a language has split into two, both can then go on to split into two other languages each. This means that it only takes 12.66 changes to create 6,500 different languages, or one new language per group every 442 generations, (or 11,000 years). Of course, this does not include languages that were created then died out, but even so, it's a lot less impressive than at first glance. In fact, what this suggests is that language transmission is actually pretty robust in the sense that it takes a myriadum (perhaps someone with better latincan correct me on this) for two sub-groups to develop so they can no longer understand each other. This agrees with Pagel, Atkinson & Meade's (2007) estimations of rates of change of single words over roughly the same time period.