It’s one of those questions that we’ll never really know the answer to.
I reckon they would have kept going until the end of the decade at the very least. Mind you, they’re both late 60s/early 70s now so probably would have still chosen to retire at the same time they ultimately did - Kay might have stayed on a little longer but perhaps not in the main role (she might have requested to report and/or read weekends, afternoons etc. on her own accord to support the transition).
I have no idea why 7 couldn’t have brought Bill in to co-anchor weekends with Sharyn, so as not to mess with their winning weeknight formula. Then again, this was not the first time they made a bad decision with regards to their presenting lineup, having dumped Pat Welsh from weeknight sport for Shane Webcke some years earlier.