Silicon Carbon sucks, it's longevity is pretty bad, and most people I know keep their iPhones for a long time and expect the battery to remain high enough quality for years.
Battery life is rated in charge cycle. Having higher capacity means lower charger cycle in the same period of time.
I'm not saying they're engineering the batteries. But they did have to engineer the whole phone to fit in a very small space without making serious spec compromises. And my statement about their engineers isn't comparative to other companies. They can all have excellent engineering teams at the same time.