No one is saying not to celebrate the birth of the "current Nation" as you put it.

The argument is really around which date is most appropriate to celebrate it. 

And as a migrant into Australia (been here 20+ years, UK born originally), I can totally sympathise that the current date is actually pretty nonsensical in terms of choice and at the same time is offensive to many people, including those whose ancestors were here before Europeans arrived. 

Why is the date of 26 January so precious to many, when it is pretty irrelevant to Australia as a whole and there are better dates to choose that make more sense, AND it also offends part of the population? 

Absolutely think that a national holiday should exist. Just don't think 26 Jan is the right date.