If they wouldn't, the Netherlands would literally not exist. Not only are large parts of the country below sea level, there is also the constant threat of flooding via Maas (Meuse) and Rhine. Raising levees is not enough, water management in the Netherlands must always look at the whole country.
I live in a country with one of the richest water tables in the world. Due to (a forecasted!) drought and lack of planning we're literally running out of water to drink. It's like Saudi Arabia running out of oil, absolutely shameful.
The Netherlands been claiming land from the sea for centuries and some areas are already well below sea level - I don't think they'll neglect their dykes any time soon.
The Afsluitdijk just got heightened and reinforced. As someone who's biked across it, I'm psyched because now you can bike on the North Sea side - previously, there was only 1 bike path on the Zuiderzee side.
With all due respect, the Netherlands is perhaps the furthest a place can be from long-term sustainability. The place is a mix of monoculture + concrete.