It has been said that if for some reason Google doesn’t launch Android 5 this year, we will have been stuck with Android 4x for more than all the other versions of Android put together.
Android 1 came out in 2008 and ICS was released October 19 2011. This means it was available for 3 years and 26 days between released.
So if we don’t see a new release before 13 November this year then owners of devices will have had 4x for longer than the other versions combined together.
While progress on a new OS is generally slow and it is an evolution so you do have to take this into account. However with it being an evolution there are less areas that have to be improved, but it can be easy to lose sight of the time frames involved.