There are many appropriate metaphors to describe just how essential a nib is to the overall experience of writing with a fountain pen.
The nib is the heart of the fountain pen. It's the engine. It's where the rubber meets the road.
Here's my favorite: It's literally the point of the entire fountain pen.
A fountain pen could be covered in 24kt gold, encrusted in diamonds, and hand-painted by Da Vinci; if it doesn't have a quality writing nib, it's a glorified paperweight.
Believe it or not, there are a lot of nibs still being made in the world today. While there are two main German manufacturers (Bock and Jowo) that produce the majority of the nibs used in many boutique pen brands, there are plenty of pen manufacturers that produce their own nibs, each with their own distinct writing experience and variety of point sizes.