It'll live a long while yet ... like TreeCo said, perhaps decades so don't euthanize it yet.
Also, your supposed to look after trees anyway so don't count that as a cost but investment into a benefit.
excerpt from
http://home.ccil.org/~treeman/shigo/CANKER.html The arborist who understands this will treat all trees, including those with canker rots and other injuries, in ways that will increase health and energy reserves and decrease stress. Easy to say. Not so easy to do. Too often the decision comes to remove the tree.