So yeah, it's a "pick your poison" kind of world and theres no easy way out. You deal with it and hope for the best. But that doesn't mean it's not worth pointing it out or that is not a problem for the Android ecosystem in the long run.
> iOS and Windows are fragmenting the market for me. They are not compatible with Android...
By that same logic Android is fragmenting it too.
> When speaking in iOS terms, you can say that 90% of all Android devices get updated once per months or so, when another update for Gmail, Google Voice Search or GCM arrived.
No, as a developer having users in different versions of Android affects the things your app can do. What APIs does it have access to and how. I don't care if the user thinks he's updated because their gmail app looks new. I care that my app needs more hours of coding to make sure it does the same on Android 2.3 than on 4.1.
> ... You can make your app run Holo no matter what Android version is installed on your customer's device.
There are some iOS developers that follow this same route. A lot of services / functions in iOS are not mandatory, you can use your own versions of it if you want to devote the time and skill to rewrite them. Loren Brichter comes to mind as a good example of this. Developers use the Apple route because it's easy and takes out a lot of the workload.