Isn't the point exactly that Google and Apple are doing it with few major screwups? And in the near future we'll see Canonical and Mozilla's versions of mobile OSes, and see hoe successful they are.
Also, their duel release model (long term support and 6 month short term support) gives them a nice test bed where they can try out radical re-designs before forcing it onto the users, who can continue using the LTS release.