My impression is that it will be hugely successful even if some of the expectations on the quality at high ISO seem a bit over the top. But shooting at 3200 and, possibly, 6400 should be OK, with careful post processing.
There are actually a lot of people who'll go for the D500: Pros getting a DX body in addition to their FX bodies, sports shooters and wildlife shooters. I know quite a few people who still use their D300/D300s for wildlife, simply because the D7200 doesn't really cut it for them, and they'll jump at the chance to get a body that allows them to go over 1600 ISO.
And if Canon doesn't manage do do something about their sensors, especially the dynamic range, I can also see some Canon shooters switching to Nikon, now that
That said, I'd always wait a few months to see if there are any bugs. I've been burnt a few times with new products already, so I'm not prepared to work as a beta tester for free for anyone over the years. I'll stick to my combination of V1+FT1+300mm for a while, despite the (fairly) low IQ and all the ergonomic quirks of the V1. And I've also still got my old D200 which is OK up to ISO 800 ...
Hermann