Ice Cream Sandwich, which is 5 years old, has about vanished.

Jelly Bean still has 30% market share, but are owners of 4 year old phones a significant portion of app revenue? I'd wager the people spending money have at least KitKat.

Lollipop, at a year and half old, has another 30%

KitKat is three years old at this point. And I still see new phones and tablets with 4.1 or 4.2 on sale.

Digital Signage platforms especially (the part of the Android market I actually care about) seem to be largely stuck on 4.1.

How much does a digital sign really benefit from tracking the most recent releases anyways? Other than the obvious issue of security fixes, it doesn't seem like that is a domain that would really care that much about trying to keep up.

Security fixes are reason enough.

Sure, but ask yourself if that's a market you want to target. Do you care if digital signage terminals can run your app?

I really care if digital signage terminals can run my apps, because I sell digital signage terminals with these apps installed.

Then make the business case to the right stakeholders that supporting the latest versions of Android is important. I don't see how the digital signage ecosystem not staying up to date is Google's fault.

I have millions of installs for my apps and my minSdk is 16, and I'm very close to bumping it to 19.

Ok, fair enough. I see now in your comment,

the part of the Android market I actually care about

Don't know if I wasn't paying enough attention when I replied, or if you ninja-edited, but that's certainly relevant context.