Except if part of the new features is a rewrite of display output that takes advantage of new hardware features / assumptions in ram characteristics, then that can be both a new feature and slow down older phones at the same time.

Then you don't include that in the build of the update for the older models it doesn't apply to, or you include both approaches to output and choose which is appropriate at runtime.

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.

Could they have saved themselves by giving users an informed choice:

"this update will harm performance but improves security and gives access to new features; your choice" (imagined release note on update clickthrough)

?