It seems likely that we will one day unite as a common humanity under 1 system of government, and virtually no regionalism at all.
When will that happen?
Not in my time, not in yours, not in my kids, or theirs, or even theirs still.
Any "unite all of humanity to live in a common brotherhood of man" movement is centuries upon centuries away.
It is just as likely we will kill each other off before this happens.
Since any globalization is in the very distant future, we need to do now what will make the most people happy in their lives today.
This means we need to accept regionalism, and other facts of life, and modify our laws to address various needs.
With an eye to the future, we should be trying to ensure that similar cultures and values are strengthened, so that if (or when) humanity unites, it is done with more of a Western-liberal democracy slant, than it is with a Chinese-communist or some other totalitarian regime slanting that system.
There, apologies for getting philosophical, but it seems that speaking of "Earth citizenship" in this day, is just as relevant as speaking about Roman citizenship.
Neither entitity exists today, we need to deal with the present, not the past or future.