DSLRBodies

I start this discussion with Nikon DSLRs. I'll get around to Canon DSLRs once I've done a bit more study and thought. 

The basic question is this? Will current DSLR models be iterated or not, and why or why not?

• D3500 — Likely no more iteration. Certainly no significant iteration. The D3500 is currently at US$500 with a lens. It's been lower than that at times. While D3500 volume was still high, even those low prices tended to work for Nikon. Over the years Nikon had been taking out complexity and parts in their low-end ILC, and at enough volume that proved profitable. As the volume dropped, that profitability started to get compromised. Thing is, you can make an equivalent mirrorless camera for less cost, even though it requires an EVF. It would be smaller/lighter and could be manufactured with more automation, both things that impact profitability. We'll almost certainly get a mirrorless Z30 to replace this DSLR model.
• D5600 — Likely no more iteration. Certainly no significant iteration. The same problem as with the D3500, only with the added complication that the mirrorless Z50 is already a better camera. The only good news is that the D5600's current price slots between a Z30 and the Z50, though I don't know for how long (see next). All the things I wrote about with the D3500 still apply here.
• D7500 — Possibly on last iteration. We need to speak of what the D7500 is: it's the great, great, great, great, grandson of the D70, one of Nikon's seminal DSLRs, and one of the key selling models for Nikon DSLRs for well over a decade. This camera sits right on the bottom of the market for the serious enthusiast user, and draws such folk to Nikon for its legacy support (oops). At US$1000, it's right at the bottom of that enthusiast price range, too. The problem for Nikon is this: partly because they spent so much effort trying to convert the enthusiast to full frame (D600, et.al.), partly due to the contraction of the market, each generation in the D70 lineage has gone down in sales. Nikon also shot themselves in the foot by removing features from the D7200 when producing the D7500. The D7500 is the lowest selling model in that once proud and best selling line of DSLRs. While I think it's possible that Nikon might take one more shot at this customer—there are a lot of D70, D80, D90, and D7000 users that never upgraded, but still might—Nikon would have to do that by adding things back into the next version and keeping the price where it is. Why? Because I'm pretty sure that Nikon is going to go with a three DX camera mirrorless line soon: Z30 (replaces D3500), Z50 (replaces D5600), and Z50 II (which adds sensor-based VR and a couple of other desired options into the older model, and ostensibly puts it close to that D7500 position). For Nikon to continue on to a D7800 DSLR, it's going to have to be The Last Great DX DSLR, and at an attractive price. Think "more D500-like features and performance." I'm not hearing any rumblings out of Nikon that they're doing that.
• D500 — Likely no more iteration. Nikon's been reading the market wrong with this camera for quite some time. At this point, the only ones who buy it are wildlife and sports shooters on a budget. That's because it's effectively a very inexpensive D5 that makes lenses seem longer. Nikon turned the D100/D200/D300/D500 lineup from a multipurpose camera into a niche camera, stopped iterating it regularly, and then wondered where all the sales went. Well, they went to FX, because that's what Nikon wanted to push. Because FX is what Nikon wanted to push, they also neglected the DX lens lineup. You reap what you sow. The sad thing is this: the D500 is still the best crop sensor camera for action. A solid-but-modest update would keep it so. With less neglect, an iterated model could have been the best crop sensor camera, period. 
• D610 — Last call. Nikon still seems to have some parts laying around to make a few last D610's, but this camera is about to be history. You can almost see it in pricing. Nikon first pushed the price to "oh my gawd that's a bargain" levels with the MB and 50mm bundle, but now the pricing is back to original. That's a signal that there's no new stock coming, I believe. Once these last few bodies are gone, the D610 is gone.
• D750 — Nearing last call. The D750 has been iterated (D780), and is currently on a serious discount that I expect to increase for the holiday, at least until stock runs out, at which point we'll be where the D610 is today: full price for the remaining few bodies.
• D780 — Possible iteration, but not high expectations. Since this camera already got all the Z6/Z6 II goodies and a few of the D6's, it's completely the hybrid DSLR/mirrorless that it can be without also resorting to a hybrid viewfinder. I'd tend to say Nikon will be happy to produce this camera as long as people buy it, but probably doesn't have it high on their iteration list, if at all, mostly because people aren't buying it (they should, it's a very good camera).
• D850 — Will iterate, and probably in the first half of 2021. First off, this is one of Nikon's most successful DSLRs. It's still selling in pretty good volume despite the mirrorless onslaught and its top end price. I'd say that the Nikon DSLR-faithful have pretty much rallied behind this camera, and with good reason, as it's still arguably one, if not the best, all-around ILCs you can buy. A Z7/Z7 II hybridization (ala what was done with the D780) should be easy pickings. I'm pretty sure that this is the camera Nikon targeted as being their last major DSLR iteration; I'm half expecting a new sensor, but probably more likely is a significant change to the current sensor (think gain settings). Beyond that, we have a few D6 improvements that can be brought over, as well. If Nikon does this right, the D880 will be a camera coveted by many. Plenty enough to warrant the cost of R&D. If Nikon does this wrong, the D860—note my subtle name change—will just signal "DSLRs are mostly dead, buy something else."
• D6 — The camera I'm most unsure of. As any D# user can tell you, the body has been near perfect for quite some time, the feature set pretty much complete. So this really gets down to whether Nikon will bring a new sensor to the game. Let's assume for a moment that Nikon believes they have to compete with the Sony A9 on the mirrorless side, and produces...wait for it...the Z1 (nope, not Z9; the pro speed model will have a different name I'm pretty sure). A Z1 would almost certainly sport a new sensor and speed. So, yes, there's the same demand case to be said for a D7 as there is for a D880: hybridization could keep this venerable lineup going as the Last Best DSLR. The reason I'm unsure is due to the timing. A D7 wouldn't be due until 2024. I'm just unsure of how much DSLR volume there will be out that far. 

So, one clear upgrade coming (D880), two possible updates (D7, D7800), and a lot of likely end-of-the-lines. 

I hope I'm wrong. Personally, I'd love to also see a D580 and the D780 get a II. But the DX lens abandonment says no to the first and the D780's price needs to come down or else everyone just switches to mirrorless at that level.