> If it were no longer possible to upload that data the system
    > would eventually become either non-functional (depending on how
    > the satellites and individual handsets are designed) or
    > increasingly inaccurate until no longer useful.

If that were to happen you could just augment GPS receivers to use more accurate ephemeris data provided from other sources.

It is, after all, now computed on the ground and uploaded to the GPS constellation, having it be provided by the satellites is just a convenience, not a necessity.