How is it planned obsolescence?
After all, this doesn't prevent people from NOT upgrading, it just prevents them from un-upgrading. It adds the ability for the company to release whatever they want as the "last" upgrade and be sure that no one (of ordinary consumers) would be able to undo it. That last upgrade COULD be wondrous and happy...or it could be a massive degradation of performance. In the former case, there's not much of an argument for forcing people to be unable to leave it. But effort was expended to make that the case, so planned obsolescence seems a reasonable conclusion.
More generically, this feature exists so the company can change the product after you purchase it and you can't undo that change. Planned obsolescence is your product not longer working "as well" as it did initially, forcing a replacement. Those concepts seem awfully close.
iOS10 is far too slow for her iPad 4, to the point where the onscreen keyboard lags severely. And many of her favorite apps no longer work.
I somehow doubt that it would be in any company's best interest to leave older devices on the market that compete with new devices.
Secondly, I've seen this happen to other iOS devices, in a clear progression.
At least when Microsoft forced people to update to Windows 10, you could still install Windows 7 later if you really wanted.
But your iPad or phone can go from being fairly usable and fast, to being slow and useless with the last update. And it almost always does.
What on Earth makes you think this?
I could have gotten many more years of usability out of it, but they nearly bricked it with an update that I could not say no to.