I'm not interested in mirrorless whatsoever.

But so many people get caught up in the latest and greatest.
(That's why they have to get the new phone model as soon as it comes out.
It's the latest and greatest.)

What will it do that your current camera will not, and do you need it?

(My phone is an S-III from about 2012 - Does exactly what I need it to do.)
(My camera is a T1i - Does exactly what I need it to do also.)
Your money, spend it if the acquisition will make you feel real good,
or if the camera will surpass your requirements.

Sliced bread definitely makes better sandwiches. If you are happy with what you have, what are you hoping to gain by changing formats? Keep in mind that the lighter and generally smaller mirrorless bodies won't necessarily give you much of a weight advantage depending the lenses you use. The other potential issues for some is how the smaller body fits the hand and the access to controls on a smaller rear panel area. Some of the entries from Canon and Nikon are not significantly smaller or lighter than their DSLR counterparts. The Canon R5 mirrorless body, for instance, is only a little smaller and lighter than the Canon 5D Mark IV DSLR. The current generation of top of the line mirrorless cameras have fixed most of the earlier issues. Read some of the independent reviews.

If you are wanting a mirrorless camera, I believe that you should get one. If, however, your D850 has not "done it" for you photographically, there's no particular reason to believe that any mirrorless system will, either.

I have moved through several Nikon DSLRs over the past 15 years. There was a specific need each time. The last change I made was about three and a half years ago...a D810 specifically to learn to do night sky photography. I needed a full frame camera to facilitate wide angle shooting, and I needed to be able to work under conditions of extremely limited light availability. The D850 enabled having a second body because changing lenses in the absolute dark is just too risky. Finally, I added a couple of D500s because I got tired of suddenly realizing that my trusted D300 didn't have some of the functions that my D850 did.

I then looked hard at a Z7. Had no reason for it. It just seemed interesting. Seemed at the time like it was about 80% of the D850 after shooting it for a few days. So I passed. Don't need all the "benefits" that everyone crows about.

If you are wanting a mirrorless camera because you believe that it contains some sort of magic powder that will advance your photography over the D850, my counsel would be to think very carefully about it. There is no magic powder.

If you have the D850, then you are pretty much on top of the technology mountain. What is drawing you to mirrorless? Staying with Nikon will be less expensive, unless you decide to get all new glass. Switching to Sony will mean new gla$$. Is there a real advantage of Sony over Nikon? Do you want to re-learn where all the controls are?

I had found that my Canon XTi did not focus in the viewfinder the same as on the sensor, and it was too old to be adjusted. Mirrorless cameras do not have this problem, and newer DSLR can be adjusted. So when I decided it was time to upgrade, mirrorless was just one more advantage. All the other new features, and being able to use my old glass, made my upgrade "justifiable".

To my knowledge, besides silent shooting and maybe slightly more frames per second, the focus issue is the only advantage of a mirrorless over over a DSLR.

This may not have been the response you are looking for, but hopefully I've given you some food for thought.

I shot with the best of pro Nikon and Canon gear (both brands) SLR and DSLR for over 40 years and then moved over to Sony fullframe mirrorless in January 2017, and have never looked back. Hey I made great and award-winning shots with my Canon and Nikon SLR and DSLR gear. And now I am still making great and award-winning shots with my Sony pro mirrorless gear.

So why switch at all? Well I could write you a book about all the advantages and features of shooting mirrorless gear that no DSLR will be able to have, including the best top DLSRs like Nikon D850, Nikon D5, Canon EOS 1DX, 5D Mk III, all of which I owned and/or shot with. But there are plenty of UHH long discussions of the particulars.

If you don't need any of these advantages to help enhance and expand your creativity and to push the envelope of your photographic comfort zone, then by all means stick with what you already have.

But if you are like many that want the best tech and brilliance of the camera manufacturer's best efforts then go mirrorless. Will mirrorless make you a better photographer. Certainly better photographic skills and knowledge matter the most, but as a 44+ year professional and a longtime University Professor of Photography, I can honestly say YES gear does matter , more today than at any time in my long tenure as a photographer.

The best of mirrorless gear allows me to be more responsive, more accurate, more proficient and more adventurous in more new ways with my photography. And every time I pick up my mirrorless gear as a pro I have a real competitive advantage over fellow pros still using DSLRs.

AP (Associated Press) just dumped their pro Canon DSLRs and moved exclusively to Sony fullframe mirrorless gear for their staff photographers and videographers Worldwide. That should tell you a lot about the state of professional photo gear nowadays, as all makers are putting their best tech into producing their latest flagship cameras as mirrorless. And all that top mirrorless tech has trickled down to even more affordable mirrorless models offered by all the major camera makers .

https://alphauniverse.com/stories/why-the-associated-press-just-switched-to-sony/

DSLR will still be around for years and there will even be some newer mid-range and lower-end models released, but the top-end DSLRs are in their final iterations now and DSLRs will never dominate again.

If you are completely satisfied with the gear you have used for years and is well-paid for, then stay within your comfort zone and budget. I would never be the one to tell you to go out and spend money you can't afford to move to mirrorless.

As a pro, I can justify the costs to make the move as a business investment and expense. For me, more and more fellow pros and many hobbyists there is no turning back the clock and being satisfied and hampered with yesterday's outmoded technology.

DSLR sales have plummeted worldwide 50% in 2019 and again another 50% in 2020. Mirrorless sales have held steady and increased their share of the overall shrinking worldwide cameras sales.

And Sony now sells more fullframe cameras (mirrorless or non mirrorless) than anyone, including Canon and Nikon. Canon sells the most cameras when you add in all categories but even mighty Canon has not caught up to Sony's lead in fullframe camera sales, and there is where the latest tech is being applied by all makers.

Use what you like and be happy, DSLR, or mirrorless. There is still room for us all.

I will always love the service my DSLRs gave me for decades, but I am NOT one to stand on sentimentality when it comes to my gear, which is just a tool after all. I will always buy and use the best tools that I can afford for the job. Cheers

https://www.facebook.com/GSWilliamsPhotography

https://www.facebook.com/groups/3048747915213474

This photo of the Golden Gate Bridge in San Francisco, California: Sony A7RIV 61mp Mirrorless camera, Sigma Art 24-70mm f2.8 Lens, 24mm, ISO 200, F 10, 1/80 second, natural light (reduced to fit here in UHH)