I think if they were going to have the character die in Doctor Who
they would have done so by now. And it's a much darker prospect than killing off the Brigadier anyway-- Sarah Jane had two fairly young adopted children who were characters in their own right, and several other young friends. Inviting viewers to imagine their grief would probably be too strong. Russell T Davies has said that as far as he's concerned Sarah Jane is still having adventures on Bannerman Road, and I think that's the right way to go.