This made me feel very nostalgic. *Drake voice* started from the Gingerbread now we here.
A really nice an intuitively easy to decode illustration of the development.
Actually when we talk about 2.3-5.1 app development there is no that big tragedy in fragmentation. All those systems beside look and some functions(that are more phone related) have no big impact on everyday usage.
I still use apps deigned for 2.3 or eve 2.1 on my xperia with 5.1.1 and... nothing... beside look they still work perfect and for most of time older version of apps work better than new ones... and still have the same functions.
Lack of optimization is really visible here.
And to be honest the same go other way. I use old phones with 4.1 and even 2.3 as a smart TV set... and all apps work on those systems... beside lags made by annoying graphic ideas(bad made material design) and lack of resource usage optimization all work fine.
So basically fragmentation is not that big deal in everyday usage and most of people just like to whining about it as a some sort of national tragedy...
It is not Gingerbread that is reluctant to leave. It's the manufacturers, who with their selling strategies are preventing newer versions of Android to reach older phones. And this trend is getting stronger every year, leading to heavily fragmented ecosystem, leading to problems for app developers and for the end users, especially those with older phones.