Whilst I agree with your basic sentiment, I cannot but feel that your argument is a little glib.
My dear old Dad was much given to the aphorism, “everything is relative”.
Compared to Japan, Britain has been ethnically diverse for millennia. The original tribal groupings were overlaid by successive waves of invaders, culminating in those new chums, the Normans.
So, whilst the Scots and Welsh and Northern Irish have long celebrated their cultural differences to their English siblings (cousins?masters?), compared to the diversity that has increasingly become a reality in Britain over the last fifty years, the British peoples have long been culturally homogeneous.
In this, Australia has mirrored the social history of Britain and North America ( slavery and its consequences being an obvious exception).
Certainly, the declaration of terra nullius notwithstanding, we suffered the inconvenience of an indigenous people who perversely refused to cooperate with their legal status and disappear. However their existence did not, in reality, make Australian culture less uniformly and enthusiastically British. We simply condemned the original inhabitants to economic apartheid and cultural genocide. Until the 1960′s, they were only included in the census under the category of “wildlife”.
The Chinese who joined the Gold rush in the 1850-60s remained as exotic fringe dwellers, as did the handful of Afghani camel drivers who helped open the interior. These people had no discernible influence on the wider culture until the appearance of Chinese restaurants in the 1960′s heralded a culinary renaissance, which delivered us from the bland uniformity of English cooking.
Despite, or perhaps because of the reality that Australia is now much more ethnically diverse than Britain, we remain essentially a xenophobic and racist country, despite John Howard’s assertions to the contrary. Evidence for this may be found in the current political dog fight about refugees.
I suggest that these are traits that are endemic in most, if not all societies.
I take your point that diversity should not need to be celebrated, however, if reminding people of the benefits of diversity helps them to offset the inevitable tensions, then why not celebrate with a vindaloo or a kebab and a Bob Marley song.
Then again, I have reached an age when I am inclined to acknowledge something as prosaic as seeing the sun rise every morning.
Perhaps it is better to celebrate the commonplace than take it for granted.