I think it is good for another two iOS cycles. However you can get it and not upgrade it.
It's going to be interesting to see what happens. I wager that the current crop of iOS devices, the 64 bit crop, will run smoothly on future iOS updates (much more so than previously; better
quality longevity). CPU performance has increased to the point where it is capable of running Apple's modern iOS features. The main limiting factor will be RAM, rather than actual processing power.
With iPads now clocking in around computer performance, they should last longer than they have historically
We can already see this with the iPhone 5s -> on iOS 10, it is still very fast and functional. it came out with iOS 7, then had 8, and 9. Its on it's 4th iOS version now, and has barely slowed down. Even the iPhone 5 runs great on iOS 10. The 4s became very slow on it's 5th upgrade, whereas the 5 is still very useable (i.o.w. its much more useable on iOS 10 than the 4s was on iOS 9)
Pretty impressive support, and to further the point, even though the iPad 2 and iPhone 4s struggle on iOS 9, are still useable. They perform well enough that you can still open up Safari or Messages or the Phone app/Facetime and use them, slower yes but you can use them. The iPhone 4 hit that performance wall with iOS 7, and that was only one gen prior to the 4s. The iPad 1 hit that wall well before the iPad 2/3.
The iPad 4 and iPhone 5 may stop receiving support sooner than later, but they didn't hit the same performance wall after their 5th supported iOS.
And the A7 devices are 2x as powerful as those; they should be buttery smooth for a while to come. RAM is the main sticking point against future functionality, I would argue. The difference between an iPad 2 and an iPad Air 2 is incredible, and iOS hasn't changed significantly so since iOS 7. If anything Apple has actually improved efficiency esp with iOS 9, and the new limiting factor is RAM.
We have not yet seen how long iOS devices can last in this current 64-bit era because of the massive processing gains that were made in the last several years, and we still compare them to the older crop of devices such as A4, A5 products that became slow later in life (but were much, much less powerful than the current crop of iOS products)