With Nikon DSLRs now aging out in every way, this is starting to pose a problem for those who wish to continue using their cameras.
First up, NikonUSA now is specifically one-year warranty only on new DSLRs and F-mount lenses (90 days for refurbished). So you don't have a long period where you can get your camera repaired free of charge for non-user caused issues. Europe and some other locales have two-year warranties or offer for-fee extensions.
Second, there's the issue of whether a product is still being manufactured or not. Here in the US, Nikon generally follows the California repair rules, which means that they will repair or replace out of warranty products for up to seven years after last date of manufacture. This is starting to push older DSLRs into "unrepairability." To wit: D3400 or earlier, D5500 or earlier, D7200 or earlier. The D3500, D5600, D500, and D5 are getting close to that same status.
One thing to note is the "or replace" clause. It is very likely that at some point a non-repairable under warranty DSLR or F-mount lens might be replaced either with a refurbished item, or problematically, with a Z-system item. The latter hasn't happened, to my knowledge, but the former has happened when parts are not available for repair in a timely fashion.
Frankly, the D5 and D500 are state of the art cameras still. In particular, the D500 is still the best all-around APS-C camera today, as far as I'm concerned, even though you can no longer buy it new in the US (gray market "new" is still available, though). So it's insane that the D500 is nearing end-of-repairability (by Nikon; a third party repair facility may be able to scavenge parts and repair it).
Moving on to older cameras, where you might eventually have an issue either to a part wearing out or an accident with the camera. What do you do when, say, your D3x suddenly needs repair? It's long past the seven year guaranteed repair rule. But maybe your camera can be repaired; only NikonUSA can tell you if they have the parts and ability to make repairs on older cameras.
My long-stated advice about repair versus replace still stands. If a repair is something other than superficial, it's going to more often than not cost US$300 to US$600 at NikonUSA to repair these days, depending upon how much tear down is involved (the only parts specific costs that tend to drive that cost higher are image sensor or digital camera board). The very first thing you should do is look at what the cost of a used version of your camera in excellent condition would be from a reputable source (e.g. B&H, KEH, etc.). In some cases, NikonUSA may have refurbished (or even new) versions of older cameras available (currently D5600, D7500, D750, D780, D850, Df). If the price of replacement is lower than expected repair cost, it's a no-brainer: replace your broken product with a new, used, or refurbished copy. Keep your broken camera around for spare parts should you ever need to have the used version repaired.
The trickier part is if you desire to "upgrade" to replace your broken older camera. Realistically, this doesn't tend to make monetary sense. If your older camera was acceptable before, repairing it or replacing it with a used version is likely less expensive than upgrading to something new. Given that your DSLR choices are limited now, it's likely that you'd consider mirrorless for any true upgrade, and then other costs start piling on top of the body-only cost.
Most people justify the upgrade option by convincing themselves that the newer features and performance are what they need. This is not necessarily true. Again, the D500: you can't get any APS-C camera as good as the D500 from anyone, DSLR or mirrorless, so you end up "buying upward" (e.g. D850 or Z8). Yes, you get "more," but you're going to pay big time for that when perhaps all you wanted was what you had to continue to work.
There's little doubt that the DSLR options are going to get fewer and fewer over time. If you're the "plan ahead" type, buying an extra body today might be a wise choice, while they're still available. Either that or plan for an eventual upgrade to mirrorless and the costs that are likely to come with that.
