If budget is of no concern, then I'd go for it. By that I mean a Nikon system to supplement your Canon system. But if you do go for the D850, you'll want to get the best lenses Nikon offers as well....that means just for the 3 mains zooms:
14-24mm - $1900
24-70mm non and VR (both have pluses and minuses) - $1800-2400
70-200mm - $2800
plus most primes worth shooting are all $2000 +/- each (24, 28, 58, 85, 105)
then you add in the XQD's at $150 just for a 128gb, which remember: will fill up fairly quick at 45.7mp.
To be honest, for most people I'd suggest just sticking within the DSLR system they're with unless they're willing to completely start over. It's annoying enough to switch between Nikon bodies (they more or less change button layout every generation and between the different models), I can only imagine the PITA it'd be dealing with both Nikon and Canon at the same time.
5DSr and 5DIV just seems like a much better pairing IMO. While I like the Nikon system (I'm currently shooting it, and will likely buy a D850 later this year if there's no issues), I can honestly say that unless you need the increased DR for your style of shooting, there's no reason to switch.