It depends on the conditions you shoot. The D8x0 including the D800 are pro-grade build quality so with take abuse and have a longer shutter life. IF you are buying new and can afford the best DSLR on the planet, it is the D850. If buying used, there are some great deals on D800 or D800e I have a D800 and use it daily with over 330k clicks and it still performs like new. If I get an additional camera, it will not replace the D800 because it does what I need, mostly people, architecture and landscapes shooting. If you are looking for new and a lower price point, the D750 is a very good camera with excellent low light performance but about to be replaced by the D760 with rumors floating around now on specs. Since it will have the same processing engine as the D500, 850 and D5,it might be worth it. Another serious consideration is the Z6 for $2000 that will, with S native lenses will have better image quality than a D750 or 760, and be a lot lighter, smaller and faster. If you have a lot of lenses now, that match the quality of the D750 or 800 and shoot mostly stills, stay with DSRL But if you are starting a new kit try the Z6 with the kit 24-70 f/4 Zmount lens with the bonus of having the best video of any Nikon
Is there something lacking in the D600? What will any new camera do that you can't do now? You are not going to see image quality differences between a D6x0 and D750.