"it is nevertheless impossible to develop another free implementation."
This is not true. It is certainly possible and Google has done so. Whether you will be getting sued for alleged patent violation however is a entirely different matter and GPL or for that matter any copyright license wouldn't be able to prevent this from happening for independent implementations. What you really ought to be complaining about is Sun or Oracle not released TCK under a license that allows independent implementations to certify themselves as Java easily and therefore get the benefits of the Sun patent grant for compliant implementations. You can also very well complain about the copyright license agreement that centralizes copyright control as has happened with many Sun/Oracle projects.
No major party seems to believe that GPL can be retroactively revoked. I don't think that idea has any merit whatsoever.