"Futureproofing" is a complete myth. You never know what may or may not work well on future iPads or not and what factors are going to decide that. Nobody has any idea what future revisions of iOS (beyond 11) are going to have in store. It may very well be that the mini 4 will continue to run well through iOS 15 or iOS 12 might such a major change that only New iPads will run it.
The point is don't worry about what might or might not happen in 2 or 3 years for a device that works wonderfully right now.
I think when people say "futureproofing" they really just mean extending the life of their product for as long as possible. I think everyone knows that ALL Apple products(as well as other companies) have built in obsolescence. Like how a car depreciates in value as soon as you drive it off the lot. In regards to apple, people always want to time their purchases with the release of new products. They prefer not to buy at the end of a cycle. Although those products are often the most discounted. Its a balance.