Furthermore, Austria was only a European superpower, and not for very long in the grand scheme of things. They are about as relevant as Prussia. Or Bulgaria. Or Sweden. Or Denmark. And only barely moreso than Hungary. Or Lithuania. It was a flash in the pan compared to Scotland, the Netherlands, even Georgia.
We all can craft arguments for why civs deserve to be in the game, but many of them, like yours with Austria, only stand up in a vacuum. In the context of other European civs, and especially with respect to Civ VI's new design philosophy, Austria is underwhelming. The only thing besides Maria Theresa it has that it can definitively call its own is the waltz. Oh wait, it can't even do that now because Australia is the waltzing civ now.
I'm not even addressing Byzantium, because I can't stoop to that level of unimaginativeness right now. Really, if you want Austria, Byzantium, Babylon, and the Celts, just go play Civ V some more.