Although you say you are defining "terminal condition", you are really defining a cost-benefit analysis to any treatment. Basically, you agree that if the proposed treatment (i) would not extend the expected life span very greatly, and (ii)at the same time adversely affect the remaining lifespan, it should not be used.
What that cost-benefit analysis recognizes is the continued physical lifespan is not the summum bonum but must be weighed against quality of that life. I think it is good that you would test any treatment against such a test. But that is exactly what Terri and others (including me) do with the cost-benefit analysis of continued physical life. If the current quality of life is zero (as was hers), unless a change can be reasonably forecast, the expected lifespan -- whatever it is -- is worth zero -- or when one's situation is as bad as Terri's, worse than zero.
In short, those of us, like Terri, who make our wishes known, wish to have that cost-benefit analysis applied to continued physical life.
Super did not assemble a 'cost/benefit' analysis.
Strictly speaking, c/b analysis has to do with measurable economic benefits.
There's no 'economic' consideration applicable when measuring the value of life. Similarly, there's no 'economic' consideration of the beauty of a sunset. More accurately, such 'benefit' cannot be measured in monetary terms, despite the wishes of the Singer/Pragmatist crowd.