I could just as easily say that the perceived decline is normal, more businesses (or registrants) are becoming smarter (savvy) to domain names and so they are becoming increasingly more aware about what is useful and useless.
I can understand that argument for raw number of domains registered, you could say, well maybe it has just topped out rather than people using something else. But the decline in .com.au in the Top 50 paints a different picture in my view.
The case study type situations aren't good either, where large companies are shifting to .com. For example bhp (largest company in Australia) has shifted to bhp.com a while ago and now hasn't even bothered to keep bhp.com.au online. There is a gradual shift taking place.
Going by those Alexa stats within 15 years it could be at the point where .com has 80% and .com.au 10%. This could easily happen if a couple of banks and organisations like Google, Ebay or Australian media organisations made the change.