> If that's the problem, then the solution would've been just adding more address space to ipv4, preserving all existing addresses and pretty much everything else about it.
That's not possible. IPv4 addresses are 32-bit numbers. Once you change that, you're no longer "preserving existing addresses".
> once enough routers are updated to support them.
That's actually "once all routers and end devices are updated to support them". As the classic spamsolutions.txt says, "requires immediate total cooperation from everybody at once".
> This could've even used the exact ipv6 packet structure, making it quite similar to v6 under the hood but more like v4 from the user's perspective.
Changing the packet structure means changing all hardware and software (yes, software too, the API for IPv4 uses 32 bits for the addresses). The changes visible to the user are the easy part.
> Instead, ipv6 created a separate world, with extra goals like making every device addressable (aka removing NAT),
It's more like "not needing NAT" than removing it; port mapping NAT is not a part of IPv4 (it's a hack on top of it), and it can be done identically in IPv6, but few do it on IPv6 since it's not necessary.
> using randomized addresses,
This is an optional thing which was added later; nothing in IPv6 requires randomized addresses, but IPv6 makes them possible by virtue of having enough free bits in the address. Were randomized addresses possible with IPv4, we'd see them happening.
> and resetting all ownership of existing blocks.
IPv6 has done no such thing; the existing IPv4 blocks are still there.