Re: "We have now run out of IPv4 addresses"
Even if they had added only one byte to the IPv4 address it would still have required a complete redesign of every network stack in the world, an upgrade or replacement of every hardware network device in the world and a complete reconfiguration of allocations.
If you are going to do something as costly, time consuming and disruptive as that you might as well fix a whole bunch of other issues with IPv4 in the process. Hence IPv6 was created to tackle much more than just address space constraints.
For instance, multi-cast never really worked in IPv4 but does in IPv6. Quite useful now the internet has become a broadcast medium of choice.