If you want to play the 'who arrived first' game, you have to separate genetics from culture. Yes, Slavic language and political structures arrived in the 6th and 7th centuries AD. But modern population genetics clearly show that Serbs and other Balkan Slavs possess a massive amount of Paleo-Balkan ancestry. The arriving Slavs assimilated the indigenous populations; they didn't just wipe them out and replace them. Genetically, Serbs are deeply rooted in the Balkans.

On the flip side, the idea that Albanians are some genetically pure, unmixed 'autochthonous' group that has sat completely untouched on the exact same piece of land for 3,000 years is a complete biological and historical myth. The Balkans have been a demographic blender through the Roman, Byzantine, and Ottoman empires. Centuries of forced relocations, economic migrations, and intermixing didn't spare the Paleo-Balkan ancestors of modern Albanians.

Both modern Serbian and Albanian national identities are relatively new constructs, just like the rest of Europe. Both groups have deep, heavily mixed, and intertwined roots in the region. Trying to use ancient, unbroken bloodline myths to justify who 'owns' what modern political border is completely historically illiterate.