It doesn't matter at all, Look at the iPad 3Rd generation it has tripple core graphics compared to iPad 2 oops it used to handle retina all together with 1gb ram.Don't underestimate that 3rd core in its SoCplus it has the same amount of RAM as the iPad Pro 9.7.
I believe the iPad Air 2 will have a similar life as the iPad 2, where it will be one of the most supported iDevices, and will continue to work well.
Even iPad 3rd generation is slightly powerful than iPhone 5 though.
iOS 15 will NEVER EVER support iPad Air 2, not even iPad Pro 12.9", and iPhone 7 Plus.No, but iOS 15 certainly will.
Even if it did, it would be it's 8th iOS Update
Do you want your Device to run slow like a snail?? and takes FOREVER to boot up?