I am sure that if you're in the middle of a long term contract with unlimited data use, Verizon will let you walk away with no penalty when this change comes into effect. But I would guess most unlimited users are in month to month contracts, and both they, and the company can terminate at any time. What surprises me is that both Verizon and AT&T allowed unlimited plans to be grandfathered in for so long.
Just to make it clear where I stand: I would love to have the option of a truly unlimited plan. I don't think the companies should be forced to offer it though. There really is a competitive market in US wireless, go to a different provider if you don't like your current one.