The D850 is enough camera to last anyone a lifetime tbh. However, GAS gets the better of us and as much as I love the D850 I am keeping a close eye on the FF mirrorless cameras. Why you might ask if I have the D850? Well the reason is travel, the D850 is just too big and bulky for me when out sightseeing and I have an EM1 for such times. A FF mirrorless camera combined with something like the new 24-70mm f4 would not be much bigger and heavier than my EM1 and 12-40mm f2.8 and so would make a good travel camera. If it then has the performance of the D850 in terms of AF, frame rate and buffer it would mean I could just run one system for everything. None are there yet, but it's just a matter of time.
The reason I mention all this is that if you want to future proof for the next ten years and you're not specifically tied into a system then it may be worth waiting and keeping an eye on mirrorless?