There is no reason to sell what you have if it serves all your needs and you are comfortable with it, and it is all paid for . There is plenty of room for film cameras, DSLRS and mirrorless to co-exist , with all still being viable choices for different tastes, different budgets and different styles of each shooter.

As a longtime pro I have more demanding needs and for me any competitive advantage was worth my move from DSLRs to mirrorless. But most shooters and certainly most UHHers are not professionals and never will have those needs.

Your D850 is the best image quality DSLR Nikon has made IMHO . More than enough for most still shooters. Where its weakness is is in video autofocus which is poor. But if you aren't focused on video shooting, that is of no concern to you at all.

Certainly Nikon's best mirrorless Z9 is made to be a true hybrid camera with superior video autofocus and performance as well as great still performance. But how many non-pro photographers really need a Z9, not many .

Use what you have and be happy until it breaks and you can no longer get parts to fix it.

As I teach all my university photography class students , photo gear is just a tool. And the individual knowledge, skills, vision, creativity, and hard work of the photographer matters the most. Using the right gear does help and does matter, but not the most.

Master your gear, but more importantly, master your craft. And be sure to have fun doing it.

Cheers and best to you.