I’m in the same boat with the 600. I will be buying a 600 in June. I’ve been debating this myself as I love my D850 and haven’t liked the Z bodies for wildlife. My plan was to buy a D6 this week and just stick with DSLR until Nikon figures out their path and produces a true pro level Z body.
That was the plan and then Sony announced the a1. I’ve had no real interest of switching brands but the a1 has checked off so many boxes I ordered one along with the 200-600 to test the waters. If it’s everything I hope it is I’ll buy a Sony 600 rather than a Nikon. I will likely hold onto the D850 and my other lenses for the non action stuff. I could see eventually switching everything to Sony but frankly if I’m using my 14-24 I’m not using a 600 so two systems shouldn’t be much of an issue.
This wasn’t an easy decision but frankly I don’t want to drop $12k on a 600 knowing Nikon is dropping a Z one next year. It’s likely going to be about impossible to get one and as of today they don’t have a body in the Z mount worth mounting it to. I don’t want to wait another 2+ years for a native Nikon Z 600 and body. So it’s likely Sony will get that sale
. Timing is everything, Nikon needs to step on the gas petal.