You have gone through as many cameras in a couple of years as I have in 33. There is no reason to upgrade or change anything until you have a photographic problem to solve. If your current gear - whatever it is - produces the results you need, don't change anything.
The Internet tends to emphasize the differences between things; in fact I'm willing to say exaggerate. If you stick to something for a while and actually try some things, you'll find that some of what is described as "impossible" is indeed quite possible. Reading the Internet, it is easy to understand that the D200 is vastly outperformed at high ISO by all the new FX cameras. And that's true. But amazingly, people have done some great work in low light with D200s and even less capable equipment. I've been told to my face that I need to use faster lenses to be successful at motorsport. (I was shooting with what the unsolicited adviser could see was a pathetically slow f/6.3 lens.) Gee, that's funny, why is it that those images are actually published?
Here in this thread, we're told that FX makes it easier to produce shallow DOF. Yes, that's true. Does it matter to your application? It's only shallower by one stop, and only wide open (since not wide open you can open up). Arguably - very arguably in my opinion - this is a good thing for portraiture, where the rage seems to be to shoot with a millimeter of DOF and let the rest of the facial features blur out. But portraiture isn't the only way that cameras are used. That stop less DOF is a disadvantage if you require DOF, for example if you're shooting birds in flight, especially jittery ones like swallows, where you may be depending on DOF to keep the subject in focus. Sure you can stop down, but you lose light doing that. It's a tradeoff. Perhaps I'm the only person who ever does macro work (he says facetiously) but I nearly always struggle to get sufficient DOF - and I find that lost stop of DOF a hindrance, not a help.
Of course, there are times when an upgrade really is required. You simply aren't going to stop action in the end zone of a typical high school football game with a D100 and a 70-300/f4-5.6 lens, not without a blizzard of noise. There just isn't enough light to shoot at f/5.6, 1/500th without ISO 6400 - or 25600. The AF probably won't lock on very well in those light dungeon conditions, either. But if that's not the type of photography that you do, don't upgrade to the gear that does it.
For all of the "it's better than the previous one," that we hear constantly, it's worth observing that even the D100 and D40 are MORE capable than most of the professional cameras in use as recently as 18 or 20 years ago, and certainly better than what was used into the late 1980s. They can take pictures as good as anything you've seen with a 35mm format until pretty much the last generation of film stocks and AF. Today we do - some - photography that's more advanced than that, but not mostly.
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!