Earlier quoted context omitted.
Another person who does not understand IPv4. IPv4 isn't a text based protocol where IP addresses are parsed like DNS. It's a binary protocol where addresses are recorded in binary and adding more address space WOULD BE A BREAKING CHANGE.
That's not what GP said. Of course IPv4 devices wouldn't be able to use IPv6 addresses, that would be impossible. But it is possible to "keep" IPv4 addresses, just make a.b.c.d to correspond 0.0.a.b.c.d.
Second, IPv6 is not just about addressing, it's a new protocol. Many things are different in IPv6, lots make much more sense. The header structure is different. Etc etc. The address space and notation are just the most visible aspects. But it's like comparing IRC and Signal. It's not just about user names, it's a different protocol.
Third, there are embeddings of the IPv4 address space into the IPv6 address space. For example ::ffff:192.0.2.128. Note the mix in notation. This is a valid IPv6 address! Perhaps a bit more cumbersome to write than your suggestion, but for technical reasons it was preferred to keep things syntactically unambiguous (that it's an IPv6 address).
Source: I work at a large router vendor, in the routing team.
Also, none of this is secret. Just read the Wikipedia page. I'm slightly shocked how a tech forum supposedly full of hackers is posting so much half truths and plain wrong information. It's all easily available and understandable, and it's not like we're discussing neurosurgery or epidemiology where we're all amateurs.