> This could possibly be achieved by ordering them to give back most of their IPv4 addresses.
That would take years and cost millions of dollars, just for the lawyers.
New address allocations make it clear that you're really only being loaned the addresses -- but the old allocations didn't. Insofar as you can "own" a bunch of addresses, the companies and organizations with legacy allocations really do own them, and I suspect many of them (the ones that haven't voluntarily given them up already) probably aren't interested in giving them up and might decide to fight back.
Look over a list of Class As and tell me which you'd like to take on in court.
Obviously they could be bribed into giving those addresses up, if someone wanted to cover the cost of switching ... but if you're going to spend the money, there are probably better things you could do to spur IPv6 adoption than spend it on corporate networks with a well-defined upgrade policy. You'd probably do better to work with the big residential ISPs, or maybe with the wireless carriers, since that's where a lot of the new demand growth is.
The legacy allocation ship sailed a long time ago, and there are better things we can do now than try to chase it down and board it by force.