While most SAW acts from the period (Rick Astley, Sonia, Mel and Kim etc.) fell by the wayside after a few years Kylie kept going, confounding most people's expectations.
Whilst I agree that Kylie confounded her critics by having a long career it's a little unfair to say that Mel and Kim 'fell by the wayside'. Mel was very ill and had to put their pop career on hold whilst she got treatment. She died in early 1990. Its hard to say whether they would have had a long career or not (Kim did release a couple of solo singles later that year) but the fact their career ended abruptly couldn't be helped.