This. When you're in a store, a thin phone feels nice in your hand. It feels new and sleek. You can't judge battery life in the store -- you need to research that separately.

You're ignoring the fact that Apple depends on a certain level of customer satisfaction to keep profits up.

The rest of the phone is "good enough" so that people don't take the risk to jump to a different OS environment on the next iteration.

Plus, Apple always promises the new iPhone will fix everything you didn't like about the previous one and people eat that up.

The problem with that hypothesis is that a non-trivial number of iPhone customers (that I anecdotely know) tend to upgrade every few years, not every iteration. If a device is "good enough" to last 2+ years, I'd say Apple is doing a decent enough job at keeping their users happy.

I wonder if there is any scope for increased battery life by disabling animations / GPU or switching to a messaging system different to Obj-C (ie, string lookups)?

Doubt it would be worth it, it's the cell/wireless/GPS that kills it. Turn on airplane mode and it lasts and lasts.

And those people eventually buy cases with built in batteries.

What a coincidence.