> We can’t re-use dots because then there’d be addresses that are _both_ IPv4 and IPv6.

How so? 192:168:1:1 is not a valid IPv6 address, so it'd be easy to differentiate with dots

If you were to interpret it as ::192:168:1:1 then it'd be a valid IPv6 address, but that interpretation would be incorrect and even so would be in the "IPv4-Compatible IPv6 address" range that was intended to map directly to the IPv4 address as a transitional function anyways.

Not really sure why that idea got dropped, it seems to me like it makes a lot of sense to do any v4 mapping in the ::0/96 range.