Well, this isn't entirely accurate. It's not like people have totally outgrown the use of stamps in the Netherlands; it's just that they've reformed the stamp system to be more in tune with the advent of digital technology. Nowadays, people in the Netherlands can buy a digital stamp online and write down its code on the back of the envelope or package they're sending. That's actually a really smart idea.
So if you're ever in the Netherlands and need to send something through the post ASAP, don't fret - just go ahead and get one of those digital stamps!