My first DSLR was Canon 20D. I loved the camera. But after a few years with it, I wanted a new toy, so I sold the 20D and got me a 40D. I loved this camera even better. At one time I thought about selling it when I upgraded my camera to 5D III. At the lowly market value, I decided to just keep it, and I still have it. After a few years of happily using the 5D III, the same digital toy lust overwhelmed me, so I upgraded it to 5D IV and sold the III. I contained my lust for the 7D III to show up, but my hope's been faded almost to hopelessness, so a few months ago I pulled the trigger for the 90D as a compromise. Of all the cameras that I've purchased over the years, however, this 90D was the only one that I had to return out of disappointment.
The point is that the digital toy lust is real AND a valid reason to upgrade, provided that you do have the funds for it. Don't let anyone tell you that you shouldn't upgrade unless you've mastered all the capabilities of the current camera. I'm now hoping for 5D V (and still and preferably) 7D III or a version of it.
But then.... if you shoot everything in "Auto" and just point and shoot, then it's another matter.... Satiating the whole digital toy lust isn't going to be as satisfying if all the newly improved manual offerings cannot be explored and utilized..... 