I would quibble about this.
Most of the value in a digital camera comes from its signal processing capabilities - there is way more money spent on developing the firmware and software in there than the hardware.
Yes.
But this is not just software. It's software in conjunction with new hardware. For example, the Z7ii has dual expeed 6 processors vs the Z7's single expeed 6 processor. You can't get the bigger buffer, higher framerate, and better low light performance without the hardware upgrade.
As for the other bits of it, yes, the initial cost is higher because of this. But look at the business model Nikon pursues from the other side of the fence.
Pretend there's a Nikon fanboy named Earl E. Adopter. He gets z10 the day it comes out wanting the latest and greatest. A year later, the new Z10 is cheaper than it was on release day, and since it has new firmware it is more capable than Earl's, has fewer bugs. Old Earl is going to be pissed if he has to pay a subscription service to bring the camera up to date when the higher serial number version is physically identical. Or, to Nikon's chagrin, Mr. Adopter will wait a year instead of upgrading now, figuring it's not worth dealing with the bugs and paying for the privilege, just run the old camera a year or two longer and buy the previous flagship after the firmware's up to snuff.
Companies like Nikon count on firmware updates because it allows them to get a body out now, even if the firmware isn't perfect (and, with Nikon, god help us, it never is) knowing they can improve it over the life of the product.
Similarly, those free updates also include lens compatibility. Which means without them, the latest additions to the new lens lineup might not perform well on an early version of the camera. Nikon sure wants us buying more glass, and mirrorless are only so perfect because of the built in distortion modeling firmware.
I'm guessing this is why it hasn't happened yet. Nikon's pretty stodgy, but most of the camera companies are conservative in their business models. They have not had a notion of plug in beyond lenses and SD cards that haven't been like slightly electronic versions of their old fashioned accessories. GPS unit or wifi as a wacky dongle that they charge an awful lot of money for.
What I'm really not seeing, is how a subscription service would be of benefit for a la carte features. If the physical nature of the camera doesn't change, someone still has to pay for the hardware AND the software to make a working product. You're not adding anything, you're simply disabling what's already there to extort a fee from the customer. So, will the reduction in cost if someone is willing to accept lower resolution, or not be able to record 4K video, be seen as a savings by the users? Or will the users who expect a fully featured camera be pissed that their camera requires a payment plan and jump ship to one of the other manufacturers who make similarly awesome cameras?
If you add more memory, I pay for that new memory, then I have to pay a monthly fee to use it? Not gonna do it. No way, no how.
From a company perspective, It's a tough balance. I'd have to see real numbers on manufacturing costs and real projections to know if it would work, but for a high end device like a digicam, there's just no way in hell I'd purchase it having to pay monthly just to use features. And I'd never buy a newly made camera if I had to pay for firmware updates, that'd just be stupid.
It's never just pay for what you use. Subscriptions are specifically designed to make you pay, even if you never use it. You'll still be on the hook every month even when the camera is on the shelf the whole month and never used at all. If NLP worked with ANY other software, there's no way in hell I'd use Adobe for anything. Same reason, some months I have it open weekly, some months I never develop a roll and don't use it at all, but I still have to pay.