I just upgraded my wife's iPad mini 2 16 GB to a mini 4 128 GB (good sale) over a week ago and she doesn't want anything larger than that so she's set. For me, I've had an Air 2 128 GB since October 2014 and I am contemplating upgrading to the 10.5 OR 12.9 inch as it seems Apple has finally (with iOS 11) given me a reason to upgrade.
64 GB would be sufficient for me, but the next step up would be a possibility as well. OR, I might just continue to stick with my iPad Air 2 and keep going over 3 years and counting. Decisions, decisions....