Why is the X Pro2 obsolete? It still makes images and still works with my Fuji lenses. The difference between 24 MP and 26MP is not really something you are going to perceive and the performance difference is also negligible. And how is the X Pro3 a better model? I have an X Pro2 because of the dual magnification optical viewfinder that works with my lenses from 14mm to 90mm. The X Pro3 has a single magnification that starts at about 23mm and has tiny frame lines at about 50mm. Yes, there are somethings that are appealing with the X Pro3 like RAW HDR files, but I can do that in post as well. And the X Pro3 adds several hundreds of dollars.
I did research Sony FF cameras when I bought my X Pro2. I did not find the image quality was superior in the Sony (I also looked at a Leica M10, but the Fuji performance was superior to that). The Sony lenses would also end up more expensive and the system heavier, which was not my goal. And now to go to Sony would leave the problem of finding new lenses, so it is hardly a bargain. Besides, I really like Fuji optics.
As far as the quality difference, there are plenty of 35mm and APS-C images here at GetPDI. Personally, I find the photographer a greater factor in image quality than simply sensor size. I do not find the Fun with Fuji X (and even Fun with m4/3) has inferior images compared with many other threads featuring FF cameras. BTW, when I got my X Pro2, I made 40" prints to see if I was going to be satisfied with the quality at that size. Since I kept the camera, that kind lets you know the result.