So I think you're saying that including a device driver in the Linux kernel solves the problem of future API changes, but doesn't solve the problem of past API changes. I.e. a device vendor who ships a driver for today's Linux would be shielded from any incompatibility with tomorrow's Linux, but there still wouldn't be any drivers for yesterday's Linux.
People usually think of the API problem as being one of future API changes, since there's nothing a device vendor can do about those, whereas the vendor could, if he really wanted to, ship drivers for past versions of the API.
But I suppose the fact that vendors don't take the trouble to ship drivers that work with all historical Linuxes is also a good reason never to change (incompatibly) the API.
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