> but the decision not to design IPv6 such that it is backwards-compatible with IPv4 is one of the worst technical decisions ever made
No. The sin was committed when IPv4 was made and not initially designed to allow for variable / expanded address space. Adding an IP Option to IPv4 packets that could carry extra address bits was not an option -- IP options aren't preserved much at all on the Internet: https://www2.eecs.berkeley.edu/Pubs/TechRpts/2005/EECS-2005-...
Furthermore, even if most routers didn't drop IP options, adding address space via IP option in a packet that old devices would nevertheless parse would have been hell operationally.
IPv6 has its flaws and its weirdnesses (multicast, mobility, slaac, etc.) might not all look so good as they did in the 90s, but the one thing that was unambiguously done right was a clear separation of address spaces from IPv4.
No. The sin was committed when IPv4 was made and not initially designed to allow for variable / expanded address space. Adding an IP Option to IPv4 packets that could carry extra address bits was not an option -- IP options aren't preserved much at all on the Internet: https://www2.eecs.berkeley.edu/Pubs/TechRpts/2005/EECS-2005-...
Furthermore, even if most routers didn't drop IP options, adding address space via IP option in a packet that old devices would nevertheless parse would have been hell operationally.
IPv6 has its flaws and its weirdnesses (multicast, mobility, slaac, etc.) might not all look so good as they did in the 90s, but the one thing that was unambiguously done right was a clear separation of address spaces from IPv4.