Well, it was a tough decision for me.
I had a 5D Mark II and I absolutely loved it, but that got destroyed in an accident, but the guy payed me some 4000 Euro (-ish) amount, so I am still lucky that i got something back. - So after it got destroyed, I moved on to a 5Ds, that was a brand new camera back then, and i was looking forward how much better it will be then the 5D Mk II.
But to be honest, even though it was a great camera, somehow the chemistry between us did not work, I struggled with it a lot, especially with focus accuracy.
So I decided to sell it. But what to buy, as I had a limited budget of course...
Canon did not really have much options, the 5D Mk III was the same with less pixels, while the 6D Mk II would have been a step back, the 5D Mk IV was not out yet. So a friend of mine told me, hey why don't you buy a Nikon? - He has a Nikon. - And I was like, come on man, for 11 years now I use Canon.
But the idea got to me...
And I looked at the D750, as it was one of the best Nikon cameras I was aware of. So I started planing with a D750, and a 85mm and a 35mm, and maybe a flash...
After reading reviews about the D750, I really liked what others said, full of positive reviews, but I felt like a step back from the 5Ds.
So I was looking at Sony too (A7R II), but it did not convince me either.
Exploring more reviews about the D750, I came across an article saying that the D850 just arrived.
So I started learning about the D850, reading some articles, some early review, its features, and I started feeling like, this is the camera, this is the camera, that could really replace the 5Ds for me.
Budget was an issue. - D850 + XQD card (that was expensive as hell in 2017) + a lens... What lens? I decided to go with the 85mm 1.8G.
But nothing else would fit into the budget, so no wide angle, no flash, no Battery Grip... I was unsure.
Finally I made up my mind, and sold the 5Ds, my 24-105, the 70-200 2.8 II, my 600EX-RT II...
And went into the camera shop saying: Hey, I would like to buy a D850 (this was in December 2017).
And they said: Well maybe in February...
I was like: What?!
And then they explained, that there is a waiting list for the D850, as Nikon cannot produce enough...
So I was thinking what the hell will I do without a camera for 2 months...
I started looking at e-bay, and other sites outside my country, did not like the idea, but I do not have a camera, and something needs to be done.
A few days later the guy from the shop called me and said: "Hey, I have some good news, some guy just dropped out of the waiting list, and we have his camera. So since you live in the same town as I do, I was thinking you could get that one."
Man, I was so happy. (I just got my D850 before Christmas.)
I can say, the camera did not disappoint. Finally I have a similar feeling, that I had with the 5D Mark II (between 2013-2015). I simply LOVE the camera.
Its images are stunning, super sharp (sharper then the ones from the 5Ds), its the single best camera I ever had, or tried.
I wrote a small comparison between the 5Ds and the D850 here, if anyone is interested: