Basing decisions on a device's "age" is a bit out of step with what is actually going on.
Even though the 12.9 Pro is 1.5+ years old, it is still the flagship tablet for "getting things done". Before anyone says, "but TrueTone", yes it is nice to have for some (I don't want it) but it's absence on the 12.9 Pro in no way detracts from it. But that's my opinion.
4GB RAM. Some brush that off as simply accommodating the larger screen. But if you do the math, the break-even point is around 3GB... that means that a 3GB 12.9 Pro would be comparable to a 2GB 9.7 Pro. The fact that the 12.9 Pro has 4GB means that it has more "headroom" than the other iPads.
Except for a few unique use cases, there isn't anything that is taxing the 12.9 Pro. Think about that. 1.5+ years later, it easily handles the latest version of iOS and apps as it did on day-1. That is pretty remarkable.
Another big "tell" in support of the longevity of the 12.9 Pro is the 2017 iPad. Apple just released that a few months ago. They're not going to make any major changes to iOS that would cause it to bog down so soon after it was released.
IMO, any updated iPad hardware will focus on providing unique capabilities (when compared to previous gens). Something along the lines of the smart connector... for example: USB C port, etc.
Besides, after all of the agonizing you did with what to buy, didn't you end up recently purchasing an Air 2?
