Another idea: Apple could pop up a dialog at a certain point saying your battery is sub-optimal (but not doomed) and you should replace it for the best experience.

I’m sure that would go over GREAT.

The battery settings do say "battery needs service" once the battery is degraded, and most people are aware that their battery is bad.

Most people don't understand that the degraded battery is the cause of the performance issues (its not very intuitive), so they should explicitely say that.

My iPhone 6S CPU freq is now capped at 911 MHz, which is half the nominal freq. Battery settings say nothing about that.

I‘ve never seen this notification once, having had multiple iPhones that browned-out at 40%.

It's not a notification; you have to poke around in Settings.

My 2011 MacBook Air has been saying this for YEARS

I know they do it when the battery gets really bad, they could move it up to happen sooner. I just imagine it would fit into the planned obsolescence narrative too easily.

My 2014 MBA has recently started saying this, but I haven’t noticed any sudden change in battery life, I still get 5-6 hours.

It’s based on a certain number of charge “cycles” and not necessarily actual remaining capacity. When I was working as a “Genius” equivalent for an Apple reseller about five years back the number of cycles before that warning would pop up was around 500.

500 seems like a fairly low number to me, I'm surprised it's not more like 1000.

That would cost Apple a lot if those people had Apple Care. Better for Apple if the minority people who perceive a problem come in and ask for a replacement/

That wouldn't be any different from now or what they do with laptops.

For laptops, as part of their hardware test they had a chart for the battery. The expected cycle count was listed in quite a few places (it was 200 for removable batteries and jumped to 1000 when they started making them non-removable). X was the cycle count and Y was the battery's maximum charge (I tried to find pics, but couldn't). The chart had a diagonal red line marked "defective." They replaced your battery if it was within the defective range, but not if your cycle count was too high.

Phones aren't any different, they're just more hand-wavy about the battery (they don't show or mention cycle-counts).

They can just exclude normal battery wear, or they could increase the price of iPhone Apple Care to include one free battery replacement and then advertise that you're basiclly getting a free battery.