[riffruff24]

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".

[hackeraccount]

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.