"When IPv4 was under development during the 1970s, it must have seemed reasonable to specify 32-bit source and destination fields that dictate approximately 4.3 billion possible addresses."
And now that those 4.3 billion addresses are moe or less all allocated and, via NAT and CGNAT, over-allocated that's many billions of nodes, some with IPv4-only baked in via firmware which need to be migrated if you want an IPv6-only world. That's a seriously non-trivial task. And it's no good saying, as one commentard more or less did some time ago, if you don't understand hex you shouldn't be using it because most of the owners of those billions of nodes don't, never will and shouldn't need to.
Unless a scheme had been devised to seamlessly - let me go full Bob here: SEAMLESSLY - migrate those it was always going to end up like this.
Would it have been feasible to devise a protocol which accepted IPv4 as a fully accepted subset? I don't know, but if it would then anything else would have been a serious mistake.