Taking the *Greek* Wikipedia article as a baseline, Dimitris Almyrantis?
I hate you.
The fact that the anthem of Rebetika, Frangosyriani, means “Catholic Girl from Syros”, is too obvious for the Greek Wikipedia page to mention; it does at least mention that the song’s composer Vamvakaris was himself a Catholic Boy from Syros (a frangosyrianaki, as he himself sings).
Ok, one thing about Syros jumps to my mind that is on the obscure side. As Wikipedia says, there are two main towns in Syros island: Syros town itself, inhabited by the Catholics indigenous to the island, and Ermoupolis, founded by Orthodox refugees during the Greek War of Independence.
Now in 1918–19, a linguist named John Voyatzidis did a dialectological tour of the Cyclades, on behalf of the Historical Dictionary of the Academy of Athens. He published his findings here:
Βογιατζίδης, Ι.Κ. 1923. Έκθεσις γλωσσικής αποστολής εις τας Κυκλάδας (1918–1919). Λεξικογραφικόν Αρχείον 6: 142–159.
Voyatzidis found that the old Cycladic dialect (which is related to Cretan) was already moribund: he could only found two or three people per island that spoke what he deemed to be authentic dialect. And this is a lot earlier than the mass dying off of the Greek dialects; but then again, the Cyclades were part of the Greek State since the very beginning.
One of the places he visited, of course, was Syros. In the old town of Syros, he heard the same thing as elsewhere on the Cyclades: one or two old timers speaking the authentic dialect, and the rest speaking a mixture of standard and dialect.
… When he went to Ermoupolis, all he heard was Standard Greek.
He published his lightbulb realisation here:
Βογιατζίδης, Ι.Κ. 1923. Πώς ανεπτύχθη η δημοτική μας γλώσσα. Ημερολόγιον της Μεγάλης Ελλάδος 218–225.
His lightbulb realisation might be obvious to us now, but I think he was actually the first one to say it at the time—so blinded were Greek linguists by the Greek Language Question polemics. Standard Modern Greek is a dialect koine. That’s why it was spoken in Ermoupolis and Athens—two towns that were essentially settled by people from throughout Greece, after Greek independence.