The word “diaspora” was originally coined for the scattering of Jews from Eretz Yisroel, and later extended to any other group of people living away from some point of origin, without necessarily any connotation of hope for/expectation of return, just a basic acknowledgement of a historical point of origin. We can now speak of other diasporas because the term was made abstract, generalized.
But now we have also a strong tendency for certain leftists to act like this original diaspora was itself a myth or a lie, as though Judaism were just a religion that spread around the world via conversion. It’s like people think a bunch of Eastern Europeans got together and were like, “I want to be one of those Christ-killers who everyone gets psyched about murdering every Easter. Let’s do it, let’s call ourselves Ashkenazi Jews. Aren’t you so excited to be a pariah and flee from country to country and die at the sword of the Christians while saying the Shema? This is gonna be really fun! I was born a good European Christian, but there’s just something about living in constant fear for my life that I really am just ready to give all that up for. So yeah, this is why we’re converting to Judaism. Dunno how that’s going to happen since there is no diaspora and consequently no one to instruct us in Jewish law, no Jews to live among, no Rabbinic court to decide our conversion is proper, nor any mikva for us to immerse ourselves in, but I guess we’ll all just wing it.”
The diaspora was not a myth. Nor is it the whole story of the Jewish people, either. After being scattered from Eretz Yisroel we interacted with the cultures we came into contact with. We learned their languages, often adding a Jewish twist to them, and over time our pronunciation of Hebrew lost sounds that were no longer familiar to us, or changed them to more familiar sounds. Our food changed. We made art and poetry and music and philosophy, as a diaspora people.
Occasionally we fell in love with people from surrounding nations, and they joined ours, becoming Jews. Still others, on a small scale, decided to join our nation for reasons other than love. This is just like how when we were still living in the Levant, lots of people were migrating in and becoming Jews, which is why even to this day we call converts “foreigners” (גרים). This is natural; migration has always been a huge thing in every area. The only strange thing Jews did was to take our identity with us no matter where we were, and consequently continue welcoming “foreigners” into the fold.