Sony has brought us better sensor technology and made Canon raise their game. Aside from that, I'm not sure I would give Sony much credit on the positive side. What they also brought us - in my opinion - is an emphasis on spec sheets rather than the quality of those specs. Specs that can be raved about by the internet enthusiasts, but often fall short of expectations in actual use. Cameras that ignore ergonomics and usability. Perhaps indirectly (although that is not necessarily so) we can thank Sony for its troll network and biased reviews that promote the overall hostile atmosphere that had inundated the camera forums on the internet. We can thank Sony for making it OK to release "beta" versions of cameras and then expecting their loyal consumers to somehow believe that having to buy the new and improved Mark II version of the camera only 2 years later is a good thing. I briefly owned a Sony A7 II a few years back and it was the worst camera I ever owned. No doubt their cameras have improved, but I won't support a company that (I believe) is largely responsible for all the items I mention above. It's a real shame that Sony has taken so much of Nikon's market share. if you are considering a Sony or a Nikon, I would urge you to get a Nikon. I recently bought a Z5 and it is a fine camera. What's even better is the new Z lenses. I thought Canon made really fine lenses (at least at the level I can afford), but the 3 Nikon lenses that I have owned are better than any similar lenses by Canon, at least in my experience.The entry of Sony into the camera business has certainly been highly beneficial to all of us, as they've been very innovative and have pushed sensor technology forward. Sony definitely hastened the switchover to mirrorless too. They have been the catalyst that has forced Canon, Nikon, Panasonic, Fujifilm and everyone else to entirely rethink their approach to cameras.
I'm not so sure that Nikon is turning a corner - they've always been at the forefront technologically - the D850 was a long way ahead of the 5DS and 5DMkiv, and is still IMO by far the best DSLR yet made. The Z9 looks very promising and could easily prove to be the "best" sports/reportage MILC, but unfortunately Nikon just doesn't have the marketing prowess of Canon or Sony. It's no use making the best camera, if you don't know how to promote it successfully.