The "experience" on an old 2.x device and the shiny 4.x isn't totally dissimilar software-wise.
Hardware-wise, there are mountains between them. Progress or whatnot.
Gmail, maps etc are quite similar. Apart from the menu button, the habits are quite similar (all apps button, widgets, press-and-hold for options, a back button which is brilliant). Ah and about the menu button: please somebody tell Samsung it has been deprecated since 3.0. Thank you. And Facebook, yes, you too.
People get new phones after one-two-three years now. And all the new phones have 4.x. It's not as if HTC or Sony or Samsung push updates to their old phones (the Samsung S2 is a notable yet minor exception).
Android (unlike iOS) does not need OS updates for key components, as most are apps that get updated all year round through Google Play (wow, what a bad name). Kernel and base stuff is important, but they are not really apparent to the end user's experience until the base apps aren't being supported anymore, which is usually a couple of years down the line AFTER the OS got a major update.