Nothing beats the look of a 400mm f2.8. I'd stick with it and find a way to keep it going. The newer gear is lighter, but also more plastic. Even the new 400mm f2.8 can't compete with the older version when it comes to build quality.

Nikon f mount prices continue to drop. I'm a wildlife photographer and am looking for a 400mm f2.8. I've found a few around 2k, non-VR. Is there any benefit to the newer Z glass? I can't spend 14k on the newer 400mm f2.8 but there is also a Nikon Z 400mm f4.5 that I've been looking at. I can't imagine the f4.5 could beat an older 2.8 in any respect, besides weight and possibly autofocus speed, not needing an adaptor etc. I'm also concerned about the older af-s motor failing and not being able to get parts for it.