It seems like it’d get increasingly impractical as the years go on to hundreds of thousands and millions of years to write them out that way, but then…I guess technically one may already do this with the preceding years, so future’s fair game for it?
Well, no reason not to add a digit after 9999.
But yeah, at some point we ought to stop using years. It’s the second that is standardized, and the year is all off. I can’t imagine we reaching 9999, but the change has no relation to the extra digit and no reason to happen anywhere near it.