It took me a while to come to grips with the decision but back in late November 2019 I took the plunge and traded in all my Nikon gear for the A7R IV and three lenses...... I was already familiar with Sony's products, having used the NEX 7 some years ago, plus a series of the wonderful compact RX100 cameras as well as the RX10 M4. I am not interested in shooting video; I'm a stills shooter all the way. At that time Nikon was just getting started with its new Z series of mirrorless camera bodies and lenses and they didn't offer what I wanted and needed, so after much thought and angsting and hand-wringing I realized that it was time to say goodbye to a brand I had been using for many, many years.
I knew myself well enough to realize that I couldn't just do this new gear thing partway -- I couldn't just buy a new camera and a couple of lenses and still plan to use the older gear, a totally different system, alongside it. I also was not at all keen on the idea of using adapters -- Nikon-to-Sony, Sony-to-Nikon, the older Nikon F-mount-to the newer Nikon Z-Mount..... No, thanks! At that point much of the Nikon gear had already been sitting in the camera bags for a rather long time as it was. I figured that I might as well go all the way with this shift to mirrorless and to a new system and so I did...... Non, je ne regrette rien.
Backing up a little here: even as I was in the early stages of thinking about shifting to mirrorless, I realized that I would also need to update my computer, which at that time was the 2015 MBP and so my first order of business was taking care of that and buying an MBP in 2018 which was much more robust in terms of storage and RAM, as I knew I'd need that no matter what kind of system I wound up buying. When I made the decision to go with the A7R IV and its 61 MP some months later, I realized that I still might have issues with those large files, but thankfully I have not. I did have to add a couple more external drives, though, and that has worked out well for me. I keep just what I'm working on in the computer, and then after I'm done with it, off the files go to external drives -- still reasonably handy and accessible but not taking up space on my 1 TB SSD internal drive. Files older than a year go to the archival drive.
As it happened, my timing was pretty good, as while I was mulling things over, changes were happening, so I waited until Nikon announced their Z series and the cameras and lenses started finding their way to people's hands; I had also initially started out thinking that if I went Sony that I'd go with the A7 III but somewhere along the line had decided that the A7R III would better suit my style of shooting and my needs.....but just as the Nikon Z cameras were coming out and I was realizing that it probably wasn't going to be the system of my dreams, then Sony announced the A7R IV, so I waited again, another period of time, for that camera to be reviewed and also in users' hands. Finally one day I said, "enough of this waiting! Just DO something!" and I did.