If you are glad and happy with the Voyage there is no reason to replace him.
Especially if you don't need temperature adjustment, listen to audio books or a larger screen.
The Voyage typeface is so clear that a lot of newer models can't match it.
1 1/2 weeks ago I bought a used Voyage in mint condition including the original red Amazon Origami magnetic leather cover (I love it!). For me, this was an excellent purchase. When the Voyage came out, I bought one a little later and the clear typeface had already fascinated me. The Voyage really deserved to be called "The real Paperwhite" because its typeface is so clear that it has a slight 3D feel. I sold mine and have never completely forgotten the Voyage. That's why I bought one again
To your question: There is no "Why" ;-)