The old (current) setup of IPv4 is one of the last, undersight, Y2K bug issues in a sense.
The designers of the system probably thought "This will end up public someday, but we will have 4 billion addresses for users. It will take a LONG time before they are all used up... if ever. After all, how popular could this ever be?"
@ GhostShip: They MAY have to upgrade again someday, but I would suspect only so if millions of addresses are being meaninglessly wasted per person or company like water in the tropics.
For those who do not understand just how large of a change this will be, IPv4 addresses are 32-bit. That's 232 or 4,294,967,296 addresses as stated in the article.
IPv6 will use 128-bit addresses. That translates to 2128 or 340,282,366,920,938,463,463,374,607,431,768,211,456 available addresses.