So for me it's not that the one I had at each time was too old, or for fun, but because the new version adds something that genuinely improve my reading comfort, while being cheap enough that I don't need to think much about that expense.
For me, the reasons went: "new," "screen contrast + first-party lighted case," "built-in light + size" in that order.
I've pre-ordered a Voyage. That's in part because of increased clarity and even thinner form factor, and in part because it frees up my PW for my gf, who honestly uses e-paper Kindles more than I do.
I like having at least one "backup" to lend to family members on vacation. I also bought a cheap waterproof-case (it's essentially a double-sealed Ziploc bag) so I just toss an older model in that and float around the pool and read worry free.
I also like supporting the product line with my wallet - and it's fun to get new toys every year or two :)