There is no real policy for replacing camera models. The companies do start working on a camera a couple of years (or more) before it is released, but design teams don't necessarily work on the next generation of the same product, so when the design work of a Z8 is done, the team may be shuffled with one or two people working on one new product and some others working on something else.
As for a Z8 mark 2, well, there will probably be a camera to replace it eventually, but do you care if it is called something else? The companies bring out what they think will sell (and afford to bring out). That's all you can say about it. For example, go back a few years and Nikon did not have a digital interchangeable camera at all. Then they brought out DSLRs, and then the Nikon 1 series (1" sensor mirror-less interchangeable lens cameras), and then Z-mount cameras. Look for specific replacement models and maybe you find them and maybe you don't. . . .