> But wouldn't that be denying agency and stripping identity from the people who actually live there now?
Indeed, that's why the founding of Israel was so problematic. Living somewhere 2000 years ago does not trump the rights of the people who lived there for the last 2000+ years. If we are to be consistent, Berlin must be returned to the West Slavs, London to the Celts, and so on... it's nonsense.
In the West we have (at least) started to acknowledge the crimes of colonialism and the various wars of conquest over the centuries. For us it is not existential, to acknowledge that what the British did during the slave trade is not existential. But for Israel it is, so many Israelis have to just pretend that the founding of Israel was perfectly just and fair, when it so obviously was an act of total lunacy when looked at through today's eyes. Please note, I do support the continued existence of Israel, because I favour the status quo, but its founding was an act of monumental stupidity.
Perhaps you mean that Germany and Austria have no right to exist, because Germanic tribes are just recent migrants there in the last 2000 years who came from the Urals or something? But wouldn't that be denying agency and stripping identity from the people who actually live there now?