Google seems to get tired of its own products very easily, and just kills them, even if they are popular.
Eg: Google Chrome on Android
Easy, just use version numbers which asymptotically approach an irrational number, like Knuth's TeX (currently at version 3.14159...ish)
But seriously, the incentive to be "done" is when a lot of other software relies on your API or file format. This is why mozillans throw such a tantrum whenever anybody tries to use their rendering engine as part of some other piece of software. Getting to "done" isn't fun.