As far as I know in Germany at least you need 5 years residence before fully qualifying for welfare if you’re not a German citizen.

A residency requirement is not a citizenship requirement. A practical example, of some relevance to my personal situation. In the Netherlands, part of the retirement system is a basic income for the elderly. The requirement for receiving this basic income is having been a resident as an adult. So a Dutch citizen who lived part of their adult life outside of the Netherlands and moves back when retiring will only receive some part of the basic income. However, a Belgian citizen (for example) who spent their adult life in the Netherlands will receive the full amount.

Anyway, the only point I wanted to make is that it is possible for EU citizenship to entirely replace national citizenship - as it already does to a large degree. And if that can be done for a continent, it can be done globally.

You just state stuff as if it’s true and must be accepted. This is just an opinion. Presumably if it were both desirable and trivial it would already be the case, no?

What an odd argument. Was it not desirable and trivial to abolish antisemitic laws in Nazi Germany?

You can probably trace citizenship back to the ancient Greeks

Instead of guessing what origin citizenship might have, why not simply look up its actual history? I can sympathize with the plight of someone who has been inundated with a bukkake of nationalist propaganda throughout their lifetime, so let me give the synopsis. Citizenship gradually emerged in the modern period in Europe and during that time replaced the previous system, which included four castes (estates): the nobility, the clergy, the burghers (from which the word "citizen" derives - citizen, city, get it?) and the peasants/serfs. Some remnants of these castes remain, but for the most part the modern citizen grew from what used to be the burgher class. Indeed, in many proto-democracies, voting rights were initially restricted to the landowning class (i.e. burghers), while peasants remained formally discriminated against. The distinction, at least formally and legally, faded away roughly around the time of WW1 (around which time many European governments also abolished the nobility, or reduced them to ceremonial roles only), and from this point we can say there is something resembling modern citizenship, and a system with just two castes: citizens and non-citizens. (Next step: a system with just one caste: people.)

This specific example was chosen by you, presumably because it was an example of antisemitism. I was thinking more like Ug and his gang in the stone age, but it doesn’t really matter. Tribes\social groupings have existed as far back as we have history, who, how and when people are excluded is varied and nuanced but not everything is a racist or bigoted action.

Yes, tribalism is as old as mankind. Yet, while you can seemingly recognize there is something wrong with Ug and his gang being bigoted against the next tribe, the nobility and clergy being bigoted against the peasantry, and Adolf and his gang being bigoted against Jews, you can't quite seem to grasp how citizenship-based discrimination is equally problematic and equally rooted in bigotry.

Some day, even someone as enlightened as myself (by today's rather unimpressive standards) will likely be viewed as backward and narrow-minded. Will we live to see it? Unlikely. The brown winds are gathering; fascism is now the most popular ideology in the West by far, and the last time this was the case things did not end well. Even so, I have some optimism that the aftermath of WW3 will induce some self-reflection on the side of humanity, and a reassessment of citizenship as a concept.