At 60 meter sea level increase we would for the Netherlands need a really big dike, but not impossible and still worth the cost.
If we would join up with Norway and the Hanseatic countries, it would be relatively low cost per country to build a dike from the south point of the Netherlands to Norway (near Bergen or so).
That dike would protect the coasts of Netherlands, Denmark, Germany, Poland, Baltic States, Russia, Finland, Sweden, South-Norway.
And if the UK has no issues to let go its splendid isolation status, for the same cost the East coast of England and most of Scotland can be protected as well.
With for example a 7 meter sea level increase there is hardly an issue.
Our existing sea dikes (that face directly the North Sea without wave breaking islands in between) are typical 12 meter above normal sea level. Adding 7 meter to that not that expensive, although it will most probably be easier and cheaper to add a 19 meter height dike 10 km or so away from the coast.
We would need to widen the Rhine bassin up to the border with Germany to handle that 7 meter higher Rhine water near Rotterdam becoming less and less going upstream.
But the maps for that are already on the drawing table. And spatial ordening directives will result from that to prevent new house building etc in areas designated as future Rhine bassin. It would also create a big green lung in the Netherlands because the Rhine becomes a waterhighway in flowing through a wide national park containing flood areas as well for the increased flooding profile of the Rhine because of Climate and the current policies of upstream countries regarding no good water retention (too much paved surface, too little forest, etc)
For now we just add in a 80 year plan up to two meter dike or changing the sea foot etc of dikes to alter the waves that hit the dikes.
Main focus now is in effect big trials for all kinds of shapes, materials, etc to increase the cost knowhow of Civil Engineering. Also experimenting with sandbank engineering (minimal tricks to have sea streams make sandbanks at the right spots for wave breaking).
If the land you protect with dikes is delivering high enough GDP (and has much capital value from houses, infra, etc)... it always pays to build dikes.
If the land you want to protect is a narrow coastal area or mainly low yield agricultural economy, it does not easily pay back to build dikes.