Longevity is a rare thing in music these days, especially pop music. Kylie certainly deserves credit for remaining popular over a quarter-century.
With the likes of Madonna (if we must compare) it was fairly evident that she had star potential right from the start. Kylie, however, was an actress in an Aussie soap and had a couple of hits for Stock, Aitken and Waterman in a cheesy format which was commonly dismissed as "disposable pop". 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. After a few years even the NME was singing her praises, which, while somewhat tongue in cheek, was at least an acknowledgement that she had transcended her status as simply a cheesy pop star and had developed into a serious contender for proper star status (helped in no small measure by duetting with fellow Aussie Nick Cave).
As others have pointed out, it's all the more remarkable for the fact that this is a female pop star, and women struggle as they get older in an industry obsessed with youth, beauty and image.
On top of that she has managed it all with a fairly limited singing voice - I'm sure she would be the first to admit that she doesn't have the gravitas of singers like Mariah, Whitney, Celine etc. so she has never really been credited with being a "serious" singer but has maintained her profile as a pop artist, something which few people, male or female, have achieved in recent decades (perhaps Robbie Williams is her nearest contemporary in that arena).