Instead of all this complexity, why not just treat phones as data devices and voice is just another IP protocol that runs on them? They had a chance to do that

Instead of all this complexity, why not just treat phones as data devices and voice is just another IP protocol that runs on them? They had a chance to do that with LTE. But the entrenched players decided instead to greatly increase the size of the specification (and the over-the-air protocol) to specially support voice (VoLTE). Granted, handoffs from 2G to 3G to 4G are quite complicated, and then throw in roaming on top of that and it gets really complicated. However, if I had a magic wand or time travel device, I'd rather have seen LTE be data only (with good quality of service support) and IPv6 only. That would have made the protocol vastly simpler, and then it would be possible to un-bundle voice altogether... eventually. I don't know that such a design would be in the carrier's best interests though, so who's to say it could ever have happened, even with magic.