Is there an obvious reason why it would not have worked to just say that all ipv4 addresses are ipv6 addresses with an implicit leading 96 zero bits?
As if IPv6 doesn’t require a full rewrite too. So basically, no there’s no reason. They just wanted to be edgy and use hexadecimal and they’ve ruined everything.
"edgy"? Come on.
And if they used decimal I bet the complaints they didn't use hex would be just as loud and just as certain, since an IP address in dotted decimal is 50% longer than in hex.
On top of that, hex would make IPv4 a lot easier to use because of how subnets get optimized. Instead of constantly rounding to weird multiples of 8 or 16 or 32 you'd only have to deal with one hex digit at a time. And in most deployments you could skip the address math entirely by sizing your subnets 4 bits at a time: /16, /20, /24, /28.
NAT64 works much better for 6->4 connection scenarios than vice versa, but 4->6 with specific connection pairs and careful split DNS is possible.