I wonder if the dutch have a plan to evacuate the whole country in say, a year time, in case walling the seas no longer feasible?

Not in an emergency scenario but more like "it doesn't make sense to stay here anymore".

Why would it no longer be feasible?

I mean, I can tell you why. It's not feasible if it costs too much. If it makes sense (i.e. the land is worth X and cost of a wall is some fraction of X that people can afford) people will do it.