The iPhone already has replaceable batteries. Making them user-replaceable just reduces battery capacity, increases size/weight, all so in 2-3 years you can save about 5% of your purchase cost by replacing the battery yourself.
iPhone users buy iPhones because they wanted the best in class functionality, if they wanted to save money they'd buy a phone with a replaceable battery.
The iPhone is a well-made phone which would have greater lifespan if the batteries were replaceable.
As it is, users who bought an older iPhone now have to pay middle-men to replace the battery or face degraded performance.