Yes we can give up our rights but what should be the standard of evidence to determine that we did, especially for the incapacitated or incompetent? On face value the Florida law doesn't seem that out of line since it calls for clear and convincing evidence (though I suppose some may argue that beyond a reasonable doubt would be a better standard). But Greer seems to have taken great latitude with the meaning of clear and convincing.
It's already set in Florida. The fact that you don't agree with the standard carries no legal water.