I dont think there is much difference between the iPad Air and iPad Air 2, I know its faster but only by about 1 second and I know it has touch id but touch id is unnecessary as you will still need a passcode to use it, so I think iPad Air is near enough the same.
I mean, anyone can go down that path. If you want to say that, you could also say the iPhone 6S is the same as the iPhone 6 except for the 0.5 second difference in speed, or the iPad Mini 2, 3, 4 are all the same except for everything that is not the same. At this point in the product cycle (Tablets having been out for several years now), the differentiation between iterations
are the features such as Touch ID, and 3D touch/Apple pencil.
Aside from that, I could also argue that while "slightly faster"/"slightly slower" may be the only difference (assuming you don't use Touch ID), if you plan on upgrading your software then you will only exaggerate the difference in speed between the two devices.
I personally don't use Touch ID either. I have never been a password person, no one touches my device but me, and I don't leave it on tables or counters - outside my home. Therefore, I would rather have the immediate unlock of sliding the screen (myself), than having to rely on the home button to unlock (and also holding my finger semi-firmly down - no matter the speed). Even with my non-use of Touch ID I still find the iPad Air 2 to be a big step up from the Air 1. With that said though, I think the difference between the Air 2, and Air 3 will be much more minor.
P.S. I didn't get into any of the other internal hardware differences, or the design/dimensions. iPad Air has the 7 (not x), while the Air 2 has the 8x (a much bigger difference than people realize for graphics). The difference in ram is
huge. However, these technical gripes are not something that everyone would recognize or run into with their normal use