Australia Day would be a great holiday for Australians to celebrate in Australia. I don't think it would qualify as a worldwide holiday though. I'm sure Australians would love to have their country celebrated internationally, but then we would have to do this for every country so no one would be left out.
It's important to celebrate all of the things that make Australia great. Australia is one of the most beautiful and unique places on earth. There is good reason that the people who make this country what it is should come together to celebrate. There are lots of ways to celebrate the country.
For the indigenous people of Australia, Australia Day is essentially an annual celebration of their dispossession and genocide. In order to show respect to these citizens it would be more appropriate to change Australia Day into a more cultural inclusive celebration in order to ensure that Aboriginal people don't feel excluded in their own country.