More than a decade ago, I allowed a knowledgeable salesman at Adorama in New York to talk me into a Nikon D200. It was a highly functional, very high-quality DSLR - among the very best APS-C rigs you could buy at the time. But, after taking it home, the term "boat anchor" repeatedly came to mind when considering its size and weight. The camera went back 48 hours later.
I'd already been doing amateur photography since the 1970's and had a few compact digital cameras by then. But this was my first digital DSLR purchase. I ended up going with Pentax, although I strongly considered Olympus's original four-thirds DSLRs. I moved into micro four thirds only a couple of years later, even while running a Pentax kit.
Fast forward to a few weeks ago, when I was in B&H's brick-and-mortar store. I saw Nikon's D500 on display and picked it up. We all know how good the D500 is. It's not just good. It's excellent and class-leading. But, again, the term "boat anchor" immediately came to mind and I put the camera down.
Yes. Nikon had better move into mirrorless for professional-grade and serious enthusiast cameras very quickly. Their low-end consumer DSLRs (D3400, et al) are fairly compact and light, so they may have a bit more time there. But they'll have to move to mirrorless in that market before long.
Nikon has been making very high-quality cameras for a very long time. And so they have deserved their success. But they've been stuck in the past... protecting their traditional DSLR line at the expense of everything else - and that's not working anymore.
Nikon and Canon both - at this late date - seem to be waking up to the fact that the mirrorless revolution is real. Canon is further along, but that could change quickly. Because in the case of Nikon, this is an existential issue. They must make mirrorless work or they may, at best, lose their independence as a company, and, at worst, go away completely.
I wish them well. But I don't envision my becoming a Nikon customer again anytime soon. It's too late for that.