No one disagrees with you, but there's not anything fun as a spectator in seeing a character die at 80% to a move with many options equipped on a character who already has more options than anyone else.
There is, because if he uses that move poorly, many characters have the potential to kill him from any percent with the following punish. The difference between a well-placed upsmash and a poorly-placed one can be very small, and even a slight amount of hesitation can be a missed opportunity.
Let's say your opponent just hit the ground and techrolled. You have to decide, very quickly, whether or not you're in position to upsmash them out of the lag. If you decide to go for it and you're right, you take a stock. If you go for it and you're wrong, though, they get to shieldgrab you out of the upsmash and potentially take one of your stocks with a chaingrab. If you pause in making this decision even for a moment, what could have been a potential upsmash becomes no opportunity at all. Maybe you
could have had a guaranteed upsmash, but you didn't act fast enough. Because of this your opponent gets to live a bit longer, and if you can't fight the instinct of going for it anyway you might wind up losing a stock of your own.
It's a character of extremes that requires highly trained reactions to be played well... again, absolutely iconic in high level play. Historically, most people have been fine with Fox's killing power as compared to, say, Peach, because Peach doesn't theoretically die every time you grab her.