alt Hacker NewsV4 address blocks, DHCP, and NAT, not SLAAC. So if I have 1.2.3.4 in ipv4, my ipv5 address is also 1.2.3.4. Ipv5 isn't a separate network from v4, it shares all the v4 state, and I can enable without giving it much thought. And once there's enough adoption, I can start handing out addresses like 1.2.3.4.5 that are incompatible with v4-only hosts.
Except you can't enable it because your software doesn't support it. What you're describing sounds very similar to a NAT64 network. You can address an IPv4 host by 64:ff9b::8.8.8.8