My guess is that the existance of Neverware Chrome OS prevents Google from making certain changes to Chrome OS, and that buying out Neverware eliminates that obstacle.
How so? It's not like Google had any contractual obligations; if they made a change that was incompatible with neverwhere, then neverwhere would just be stuck, wouldn't they? Unless you mean that Google intends to do something user-hostile, but then there's nothing stopping another Chromium OS derivative from coming along.