It's interesting reading everyone's perspectives. There isn't a "right" or "wrong" answer really.
My personal journey was a little different. I started with Canon in 2008, ran into a financial difficulty and had to sell my gear around 2013. When I got back into photography, I was basically starting from scratch and my limited funds and trends at the time steered me towards the original a7. I was tired of the Canon sensor tech lagging behind the competition and was curious about adapting vintage gear, so I had a couple of fun years adapting old manual lenses on the a7 when that was the newest trend.
Around the a7r III time, I was over the vintage lens phase and wanted to rebuild a good AF based kit for general purpose and wildlife. At that time, there weren't good options for wildlife in Sony due to low pixel density on the a9, so I added a D500 and 200-500 for only wildlife. I really enjoyed this kit for a while and then was an early adopter of the 500PF when it was released. This was revolutionary at the time and one of my favorite pieces of kit ever. Something about the D500 and the 500 PF made the perfect wildlife kit for me with great build quality, perfect (for me) ergonomics and fantastic image quality.
I eventually tired of running dual systems and it made more sense to steer towards the Sony end since it served my non-wildlife needs better. I picked up the a7rV and the Sigma 500, and have found a kit that I like going all Sony now. I am a little jealous of a few Nikon offerings in the wildlife space (mostly the PF lineup.) If I ONLY shot wildlife, I'd shoot the Z8 with a 600PF and call it a day. I love those ergonomics for wildlife, but I prefer my a7rV and the Sony lenses outside of wildlife shooting for their smaller size to performance ratio.
