p.5 #1Why would you choose Nikon over the other brands?
I recently made that choice, having been a (hobbyist) Pentax shooter for a number of years, and fallen off in photography for a while. Recently decided to rekindle the interest while helping my wife with art repro and had to get caught up with the shift to mirrorless, advances in the gear. Nikon appealed to me in part because it seemed pretty straightforward to adapt some of my great old Pentax glass for MF use, but also because of the brand history and vibe and cool options like the Zf (which I bought). Also just feels photographer-focused somehow in a way reminiscent to me of what I liked about Pentax. The others all have their unique appeal too but that's the direction I went and I think it was a good choice.
p.5 #2Why would you choose Nikon over the other brands?
When it was time to move to mirrorless for me, the Z8 was the first camera to convince me. First serious camera with no shutter, i.e. no sound, no vibrations. Great AF and tracking, decent size (not too small/large). The new Z glass is great, but Nikon still allows a nice mix (new/used) of previous F-mount glass. You need to have the 1.4/58. If you appreciate long teles, the PF 500 with adaptor works perfectly. AF speed of the Zii 24-70 / 70-200 is awesome as are size and weight. Sum up this gear and 10k are quickly spent. PF 500 should get an PF 800 as friend at some point. In addition I can use all kinds of other/older glass that I own or want.
Nikon has a smart product management that uses resources visely. TC-tele lenses are extremely useful for photography and the company does not have to present too many lenses with similar specs. Some of the latest F-mount glass works nicely with adaptors. PF tele glass is still unique (I don`t listen to Sony). The PF 800 has the largest front diamater of all PF lenses available - large enough for a 4/500 in the future. Do I need 1.2/1.4/1.8 options for 35/50/85 mm lenses, nope, but it is noce to have a choice. 24-120 is much more important for everyday use.
There are also things I don´t like that much about Nikon :-) - but overall I am happy where I am.