Actually, the 4S saw production and sales continued in India all the way up to February of this year (2016), which would fit the "only currently sold devices" pattern, if applied to a global scale.
Though it's true that the 4S and iPad 2/iPad mini are similarly specced as well, they've demonstrated that other similar specced models don't get the update while others do. See the original iPad and iPhone 4.
What you've failed to realize is, the first iPad and iPhone 4 were not similarly specced. Sure, they both had an A4, but the original iPad had a mere 256MB of RAM while the iPhone 4 had double that.
In fact, it's that tiny amount of RAM that was likely the reason why the original iPad didn't even make it to iOS 6 while the iPhone 3GS, also with 256MB, did make it. It's less taxing to power a device with a 480x320 display with 256MB of RAM than to power a 1024x768 display with the same amount of RAM. The 4th gen iPod touch, however, did have to power a 960x640 display with 256MB of RAM with iOS 6, but… 960x640 is still a lesser resolution than 1024x768.
Speaking of, though, this is why the iPod touch 4th generation didn't make it to iOS 7 while the iPhone 4 did. iPod touch models always get screwed in some small (but important) way hardware-wise, to save costs. In fact, Apple knew they weren't going to support the 4th gen iPod touch with iOS 7, which is why they introduced that low-cost 16GB variant of the 5th gen iPod touch (the one with the cheaper quality buttons that lacked the iSight camera) in early 2013, just a few months before WWDC 2013.
Regardless, it's been confirmed: Apple is officially done with the A5 chip. They had a bit of an error earlier today on their site, saying that iOS 10 would support the iPad 2 and 3, the first iPad mini, and the 5th gen iPod touch, but this was clearly a mistake (because Craig Federighi's slide in the presentation did not include these devices), and Apple has already corrected it.
iOS 10 will support:
- iPhone 5 and later
- iPad 4th gen and later
- iPad mini 2 and later
- iPad Pro
- iPod touch 6th gen
(So it looks like I was wrong, too. The A6 is here to stay for another year.)