Bramalea City Centre has finally found new uses for the ex-Eaton's/Zellers/Target space. Losing Sears will be a blow, but it will survive as it's Brampton's only good mall. More vulnerable will be malls like Woodbine Centre, which has been struggling for years, and smaller town malls like Timmins Square and the Sault's Station Mall, where Sears is the anchor, and where the possible replacements (Wal-Mart, Winners, etc) are already in nearby big-box centres.
In the GTA, the only troubled Sears mall that really comes to mind besides Woodbine is Promenade. But it's in a desireable part of York Region and there's rumours of possible redevelopment there.