Unless the Wights simultaneously continued further south while attacking Winterfell, there is a fraction of the North's power left in their southern realms. I'm not too knowledgeable about AWOIAF outside what's shown in the series and ACOK mod for Mount and Blade but there is a decently sized city in the North's southern realms.
However, since Cersei does have naval support I do agree it's looking bleak. Without the fleet I'd say the North has too good a castle to hold as Doc said many pages ago.