Hindsight is always somewhat easier than foresight (neither are easy, though ;~).
A common theme started running through everything I heard from photographers in 2019: "I've got enough of what I need."
To be clear, that's a modern 20-24mp sensor in a mature, full featured camera that has a full set of lens choices. Note that I didn't write 61mp, or medium format (or even full frame), or DSLR/mirrorless, 16-bit data, 5K+ video, or anything else. At 24mp you've got 6000 pixels across the long axis, good for a 20" print. You have very good pixel-to-pixel noise handling (even down into APS-C, and some would argue m4/3).
Nobody wants to rebuy their lens set. Nobody wants to spend US$1500+ on a camera that needs more refinement because it's a first generation model of some sort. Everybody is also balking at some perceived liability of the new (Sony ergonomics, Nikon tracking focus, Canon old sensor tech, etc.).
On the Nikon DSLR side, if you have a D500, D850, or D5, what more do you really need? And are you really willing to pay another bi-annual Body Tithe to get it? I'd argue that most of those D5-generation users would be more interested in a lens that they perceive to be missing. For the D500, that would be a whole arsenal of lenses, but even for the D850 and D5 we can come up with lenses that we'd like to see.
The camera makers like Nikon never quite got out of the Japanese consumer electronics "box business." This is the business where you make a high end box at high price initially, trickle features/performance down to build a full model lineup, and then iterate modestly, just enough to entice two-, three-, or four-year upgrade cycles from users. Thing is, the people still buying cameras aren't interested in low end stuff, and smartphone users aren't interested in low end stuff, either. So the whole box biz just slowly crumbles.
Meanwhile, the high end user who might still buy isn't for the most part seeing anything compelling, which stops them from buying, too. As I've pointed out, Nikon was so determined to put their new mirrorless line out in a way that didn't immediately make the DSLRs irrelevant that they positioned the Z7 under the D850 in terms of performance and features, and they've done the same with the Z50 versus the D7500. (The Z6 is an interesting anomaly; we won't know exactly where it was positioned until the D750 update comes.)
Canon was worse. While Nikon targeted a high prosumer, but not quite the highest prosumer, Canon targeted the entry consumer. Five years ago, a US$1300 RP would have been an immense hit. Today? It's a flop. It's actually a nice entry full frame camera, but old sensor tech, low performance specs (e.g. frame rate for action), and a host of other obvious "you didn't need that" feature removals didn't exactly send a compelling "you need this" message to anyone.
I've been writing this for many years now, but it's starting to become incredibly important to long-term survival: the camera makers need to get closer to their customers, fully understand what motivates any additional buying, and stop playing their cards so close to their vest in feature sets. Nikon didn't really do any of those things. Canon didn't really do any of those things. So in 2019 Canon and Nikon didn't sell a lot of product, and what they did sell they had to drop the price on in big ways to move it.
Sony's in a slightly better position temporarily, but mostly because they went through a similar problem earlier. But even Sony is having trouble convincing people that the latest and greatest is worth picking up now.
