What happened to the Greek population of South Italy?

I refer you to Nick Nicholas’ answer to Why does Grecani language not exist in Sicily (Magna Grecia)?

From what I’ve read, the Greek-speaking population of Southern Italy gradually shrank geographically. It was quite a broad area in the 1600s; it was a much smaller area in the 1800s; and it’s pretty tiny now. In Calabria, it may well be completely moribund.

The Greek-speakers didn’t vanish into thin air, of course; they assimilated. The elimination of Greek rite Catholicism in the region would have expedited that. And you’ll notice how careful Greek scholars are about describing them as Greek-speakers, “hellenophones”, rather than Greeks. In all the cultural ways that matter, these people are Italians, and have been for a while.