The Kindles had been in production for years before Amazon adopted the "generation" terminology.
They where just running out of consumer distinctive marketing names for the models.
Such as:
There had already been five models built and sold with keyboards before the marketing people named a model: "Kindle Keyboard".
No, it is not like a (general purpose) PC operating system.
These are (specific purpose) embedded operating systems. Each built for that specific purpose. Were here "purpose" is more defined by consumer features than by changes in hardware.
The last (OS significant) hardware change was the adoption of the i.MX6 series of SoC chips (at the PW-2 point of evolution).
There are two reasons at work here for why new firmware versions are not backported to older models:
*) Planned obsolescence.
How can you expect to sell "new" models ("generations") if the currently promoted features are made available on the older models?
To further that thinking, why not at the same time take advantage of some people's search for "bragging rights" ? I.E: They must have the model associated with the next larger number (generation number). And electronic device (or service) version of referring to things like automobiles by "model year".
*) It is expensive to support multiple models with vastly differing hardware resources with the same firmware build.
So making the cut off point any device with an older than i.MX6 SoC (System on Chip) device is rational (not a typo, I intentionally avoided: "reasonable").
There is a similar reasoning (and reasons) going on at the next higher level, the chip manufacturer level.
NXP (formally: Freescale, formally: Motorola) has dropped or will be dropping support of any chip model prior to the i.MX6 series.
And it is a series, it is just that Amazon only needs to use the low-end device to build an e-reader. The others are designed with (computer) resources more aimed at tablets, cell phones, and single purpose controllers (like are in your car or kitchen mixer).
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There, probably more than anyone ever wanted to know about why the Kindle Touch is still running the 5.3 consumer interface feature set and the original Paperwhite is still running the 5.6 consumer interface feature set.
Different system chips requiring different firmware build systems.
Everything newer (so far) are all i.MX6 based devices.
See? I even (finally) got back to the thread topic.
But they all (except maybe the K1) are running Linux.
And here we have a small scale version of why computer systems manufacturers get a little bit fuzzy on their marketing driven hardware choices.
For instance: IBM would rather their customers forget that IBM-360 devices run Linux, just like the Z-Systems do (or can).
Why would they want to encourage people to spend thousands on a small, used, 360 product model that serves the buyer's purposes when they can encourage people to spend millions on a "modern" Z-System?
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The "bragging rights" numbers game:
http://www.nxp.com/products/microcon..._HOME?&tid=FSH
Of course they inherited the little problem that the i.MX6 is bigger, better, newer than an i.MX53 product.
But you can tell by the layout of that page which direction their marketing people are looking.