Given the actual changes apple+samsung made it's hard to argue this was in any way necessary - they chose to prioritize making the old phones look more like the new ones, which makes sense from their point of view because they want to encourage upgrades - so maximizing (visual) consistency at the cost of sluggishness is much better than maximizing quality for old phones at the cost of making phone differences more apparent.

And in any case: they're free not to offer feature upgrades to old phones; I just doubt that's a wise choice in the market. If they really can't provide a few updates and reasonable performance - good riddance, their successors surely will.