One thing about technology: it eventually makes the average product capability rise above the actual needs of the average customer.
Consider automobiles for a moment. I'm still driving a 14-year old vehicle, and that vehicle is in fairly pristine condition. For basic transportation needs, it well more than suffices. Are there features my vehicle doesn't have? Sure, with no backup camera probably being the most desirable missing bit (and which can be fixed by third party add-ons).
In cameras I still encounter quite a few folk, including pros, using 10-year cameras. Certainly 7-year old cameras (on the Canon side that goes back to the 5Dm3 and 6D, and on the Nikon side that takes us back to the D4 and D800). I'd tend to argue that DSLRs of that age or newer more than suffice for the average customer.
When you get to that sweet spot—where the camera is most of the time beyond your actual real needs—you tend to enter into what I call Last Camera Syndrome. By that I mean you're no longer in the market for a new camera unless (1) you lose or break yours; (2) a product comes along that clearly fixes your biggest user problem; or (3) you succumb to the onslaught of marketing messages that attempt to get you to lust after the new.
Let's discuss this in reverse order.
Canon and Nikon's DSLR problem is #3. Both companies have their issues with this. I can't really say Canon has been delivering new DSLRs that really are lustful. Most people seem to be perceiving them as modest iterative updates. That means that it is primarily marketing messages that Canon can use to provoke buying action #3. I'm not convinced Canon is doing a good job of that.
Meanwhile, Nikon has had some extremely nice new DSLRs that don't seem like a mild iteration, particularly the D850. The D850 is indeed a lustful piece of gear. I've called it the "best all around camera you can buy" pretty much since the day it appeared, and nothing's really changed in that respect. Nikon's problem, as I've outlined before, is that they're not all that great at marketing messages. (Not that I'm asking them to quote me. But it sure seems like my messaging about the D850 has been stronger than Nikon's ;~).
So #3 isn't really getting the Last Camera Syndrome folk to move old DSLR gear out of their closet and replace it with new. Price is basically the only motivator Canikon have now, and they have to be careful with that lest they hurt their mirrorless product sales.
Meanwhile, I'd say that the primary user problem (#2) that many folk desired to be fixed was size and weight. That's partly because the typical ILC user is aging. Carrying five pound necklaces around all day isn't fun (with another 10 or more pounds on your back), and Medicare doesn't offer any quick fixes ;~). While Canon and Nikon both have smaller, lighter DSLRs to sell—in particular the Canon SL1/2—they fumbled at marketing those (#3 again!).
Since mirrorless did offer to fix the size/weight issue, you can see why both Canon and Nikon hit the full frame mirrorless market with reasonably small and light models, and why Canon tackled the crop sensor mirrorless market with near compact-sized cameras. But that doesn't solve the DSLR problem, does it? All it does is try to keep folk from switching brands when they opt for one of the user problem solving aspects of the mirrorless cameras.
Finally, we have #1: what are you going to do when you drop and total your D810 or 5DmIV? Canon will say "replace it with an R" and Nikon will say "replace it with a Z7." Why? Because there's the upside of new lens sales, too.
Recently we've seen Nikon executives trying to talk their way through the dilemma. In a dpreview interview at CP+ they were quoted as saying they "want to grow the [mirrorless] and [DSLR] series at the same time—we're not weighing one against the other." That seems to be a message that everyone in the Nikon marketing and sales side is promoting, whether at the corporate or subsidiary level. So it's clearly a talking point that's been communicated from on high.
Despite everyone constantly talking about the demise of DSLRs, the reality is that Canon and Nikon DSLRs in 2018 still outsold the entire mirrorless market by more than 50% (unit volume). People still buy DSLRs, and that won't go away for a minimum of four or five years, probably more (you can still buy a Nikon F6 film SLR today, almost 15 years later).
The problem is simple for Canon and Nikon: they need to make you want a new DSLR while at the same time make someone who's decided on mirrorless want a Canon or Nikon mirrorless camera. That's a tricky little problem to have.
How well they solve that problem will determine the long-term market shares of the camera business. Canon says they want 50% or more (actually, they use the words "capture as many users as possible;" note the word "capture"). Nikon wants to stay in the 20's and Sony's wants to get there. You'd think those market share desires would compel some really hot new products you might want. To some degree, the R and Z and the A updates are that. But where's the last DSLR sizzle? It can't be the D850 can it?
