Technically speaking, they could have designed the battery differently so it's much more likely to last the full service life.

Acting this way was a choice, which may have been fine with had Apple not denied the issue and allowed replacing bad batteries under warranty.

I'm pretty sure they ran an exchange program for affected models of the 6s: https://www.usatoday.com/story/tech/2016/12/06/apple-says-ip...

There's only so much you can do to make lithium batteries last forever. These days the phone tries to figure out when you need it charged by in the morning so it's not sitting on the charger at 100% for eight hours overnight, and this is supposed to help.

That replacement program was for a different problem, not battery degradation per se but a manufacturing error; The throttling affected other models too (and replaced 6s batteries after enough time I guess).

It's true that battery degradation can't be avoided eventually; but there's a reason why these complaints are rarer on high-end Androids (Also low-end, but it's likely users have appropriately lower expectations for these).

The battery degradation is inevitable though.

Apple doesn't manufacture the batteries. And you can't magically make batteries that perform identically. Every battery made has variation in it and that variation can't be seen until it starts causing failures.

The change would have been to simply advertise the service life was shorter than it actually was, to account for various worse batteries, but the long tail of worse off batteries would have still been there regardless.