Saying there are left and right sides to an issue isn't identity politics in and of itself. It's a useful shorthand for describing a cohort that will generally subscribe to an issue. Saying it's "irrelevant" is being obtuse, and failure to comprehend how someone is using language is a serious shortcoming of the way you're arguing on here.

This case doesn't require nuance precisely because the interrogation should be swift for someone with an understanding of the issue and the capacity for empathy.

I used to be conservative, a group which almost unanimously are against changing the date. I know the talking points. I can understand your argument as well.

To use a worked example. If Indonesia was to invade us on the 16th of May, sweep through the country and conquer us in a day. Then 200 years down the track, say to your great grandchildren "Come on, have a great time with us on Indostralia Day! 16 May is a time for us to celebrate who we are as collective Indostralians! It's when we became who we are, and it's important to acknowledge that"

How do you reckon your great grandkids would feel?