Want to free up 90% of the IPs? Easily? Then charge for them!!!
If MIT had to pay $1/mo for each IP address ($16 million per month), it would immediately give most of them back.
If MIT had to pay $1/year for each IP address ($16 million per year), it would immediately give most of them back.
And yet for people like me, paying (in round numbers) $50 / month for Internet, $1/year or even $1/month would be "in the noise".
So how many hundreds of billions of dollars will instead be spent on the designed-by-nerds very-difficult-to-implement not-very-backward-compatible solution of IPV6?