Why do people continue to call it the Summerset Isles? It was renamed Alinor.
Probably because that name-change was initiated by the Thalmor, and they've got a tendency to tick people off. So people use the old name out of spite.
Not a fan of the Thalmor myself, but I admit that Alinor has a pretty good ring to it, sounds more elvish.
OT: I doubt they'd put the entire location in a DLC, we probably won't defeat the Thalmor until the next game at least. We might get a DLC to interrupt more of their plots though or act against them in Tamriel, I'd be happy to see something along those lines.