http://borg.uu3.net/~borg/?ipv6
It basically means, you grab IPv4 implementation, extend address space to 64bit, call it IPv6, vioala. Problem solved.
IPv4 was battle proven for years. It mistakes were sorted out (CIDR addressing, ARP poisoning, DHCP/BOOTP, etc). It was widely understood. NAT is okey, CGNAT is not. The only problem was the address space. And they failed it...
123.123.123.123/48 or whatever would be completely useless and dropped by an incompatible router and a server wouldn’t know what the hell to send its packets back to if it even got a response from something with that.
By reducing the amount of work needed in order to get the new thing implemented. IPv4 is super simple by comparison to IPv6. I'd say there's at least an order of magnitude more complexity and legacy in IPv6 than in IPv4. Just look at the number of IPv4 vs IPv6 RFCs.
> 123.123.123.123/48 or whatever would be completely useless and dropped by an incompatible router [...]
It would still have been a new protocol, just able to share a lot of source code with IPv4.
Of course you probably never worked with source routing in IPv4 - I spent a lot of time testing it because marketing couldn't prove none of our customers used it. Even then nearly all routers shipped configured to drop any packet with source address routing, but to comply with the rfcs we had to make it work.
The only place where source routing is used these days is MPLS-SR aka Segment Routing. The edge router precompute the path and sets all labels. Core routers just pop those labels and switch.
Most of time you just use normal routing or ocassionaly PBR (Policy Based Routing) with is not source routing. Src IP can be one of the parameters to decide.