This doesn't mean IPv4 will disappear. Just like the 20+ year old machines that are still on the net via IPv4 -> legacy protocol gateways, pockets of IPv4 may exist for decades via similar devices -- but at that point, we just dismiss those guys as crackpots.
Maybe not quite crackpots, but you are right that a sunset date is really a marketing device, and if done as a manifesto, would gain a lot of publicity. If the manifesto has a clause that allows a signatory to keep running IPv4 for specialist purposes that are not core to the public Internet, then what will happen is that the public will force the sunset to happen. But behind the scenes people will still be using it just as they are still running X.25 networks today, out of the glare of the public eye. For this to work you need a team of sensible people to put some effort into crafting a workable manifesto that network operators would actually be willing to sign. 2019 seems like a date the people could actually commit to, in fact even 2016 may be workable and is perhaps desirable because it will be within the planning horizon of a lot of folks starting next year. --Michael Dillon