Having it optional for people who want it, trading off design and durability? Sure.
I'm perfectly happy with the iPhone's battery life, never intend to (nor wanted to) change the battery, and use it around or even under sea/pool. This "improvement" would either change the IP rating or lose a lot of inner space to keep the waterproof rating while still supporting battery changing.
Why would I, as a user, want a totally backwards rule for something I'm never planning to do in my entire life? If someone wants that functionality, they can get a cheap Android phone with swappable battery and 100 spare batteries. Making such a thing mandatory is beyond ridiculous.