the key difference here is that qantas is replacing types and doesn't have aircraft in it's fleet long term that will do the same job.
conversely, ansett "collected" types, 737 and a320 was an unecessary duplication of resources, spares etc, and it hang on to the 727's for way too long, getting rid of the last in 1997.
there is a clear difference between qantas, who have different types of the same aircraft, and ansett who had similar aircraft types deployed onto the same types of routes.