ICANN is already preparing for secondary markets to appear where people can auction off unused IP space. It's inevitable and they know this. Eventually everyone will migrate to IPv6 simply because the addresses will be free.
There will never be a day when you cannot get a new IPv4 address. But there will come a day when you don't feel like paying for one.
They aren't QUITE free. ARIN charges yearly fee of $1,250 for a /40 of IPv6 space. Of course, that's theoretically 309 SEPTILLION addresses (2^88), as much as organization will currently need. $1,250 for up to a /20 of IPv4s, which is a paltry 4,096 addresses.
There will never be a day when you cannot get a new IPv4 address. But there will come a day when you don't feel like paying for one.