I don’t think anybody really cares about an extra half millimetre of thickness, especially if it means that you can save hundreds in replacement costs and extend its life by a few years. Nobody’s buying an iPhone and busting out the calipers to compare it to their previous phone.
Apple’s sales fall and people don’t buy new phones because “it looks just like last year’s phone.”
You also start running into usability issues. There’s only so thin a phone can be before it’s less of a phone, and more of a blade that’ll bend if you sneeze at it wrong.