Failure in given Generation = Failure in comparison to its competition =/= Terrible system
First two are objective, the last is subjective (unless in the context of hardware which can be scientifically analyzed. In terms of games library, it is subjective).
EDIT: The fact that Gamecube's hardware isn't crippled by terrible restrictions is also worth noting - development for the N64 was a nightmare even though the system was pretty beefy, that was not the case on the Gamecube, so points for that go to the purple cube.
You don't have to discuss it with me - actually, you brought it up yourself today. To be perfectly frank with you, I know more "good" games for the Gamecube than for the Nintendo 64 and I'm entitled to that opinion because it's subjective. That being said, it's completely besides the point - my personal tastes and whether or not I like a given console should not cloud my judgement when we go down to numbers.
The overall "score" of a system should be derrived from numerous variables, not just "sales" or "profits".