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.
I have been happy with lens choices, particularly the Zeiss offerings. This is now my main 50mm (Zeiss Planar T) that destroys anything I put on a Nikon Z (for my purposes) in terms of rendering. Yes its chunky but so is the Viltrox 50 1.4 which this lens blow away, and the Nikon 50 1.2 which is huge. I know the 50 1.8S is smaller and sharper but rendering is important. For the classic Zeiss look, which I love, there is nothing better for a A7Riii.
This goes along with the 35 1.4 Distagon and the 135 GMaster, and the Viltrox 85 1.4 (a no brainer lens if there was one)
Bottom line,between the better AF and these lenses....no regrets switching from Nikon. For the money I spent on these 4 lenses and the camera, the bang for the buck is phenomenal.
I have run two MILC systems for over a decade with Sony being the newcomer. I recently invested further into Sony with the A1 and two OEM radio flashes. Fixing the godawful A7ii menu layout was a key motivation. Both systems have plausible bodies but the lens offerings continue to distinguish one brand from the other: Plena, 24-70/4S, STF, 70-200 G II, etc.
