Even if it is the player's fault for SDing or getting gimped, it doesn't mean you should already be 50% of the way to lose the whole match. I'm not saying you shouldn't be punished for making that kind of mistake, but it is too harsh of a punishment for that. If ZeRo lost a match thanks to accidentally SDing early in his first stock, even if it was his fault, how do we know he wouldn't be able to make a comeback if it was 3 stocks?
And yet, I've seen countless comebacks in a 2 stock environment. "Comebacks are easier" isn't a clear cut argument for 3 stocks vs 2 stocks. In practice, I see them happen all the time.
I hate that people parade this argument around. It's simply not true. . . Well, to expand, it CAN be true, but it depends on player type.
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2 Stocks:
A hypothetical Player is down a stock early. It was a quick gimp, and the opponent is at an extremely low percent.
Player A might think "I've only got to take 2 stocks. I can do this"
but
Player B might think "I'm down 50% of my stocks, this is worthless" and basically throw the match.
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3 Stocks:
Same scenario. Down one early stock, opponent at a low percent.
Player C might think "Only 3 stocks to go, I've got 2, I can do this" and still win.
but
Player D might think "That went poorly. I've got to face this for another 8ish minutes? And still have to take all 3 stocks? No thanks" and basically throw the match.
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Players A and C have the right attitude to make a comeback. Players B and D, had the stock count been different, may have had a comeback in them -- but they didn't, and faced with the numbers, they defeated themselves.
Player C, in 2 stocks, may have responded like Player B. Player D in 2 stocks, may have responded like Player A. Basically, saying "3 stocks is mathematically superior for comebacks because you have an extra stock" is a completely fallacious statement because it fails to factor in the player psychology aspect. It CAN be true, but it can also be equally as true to say that a comeback only happened because there WASN'T a third stock. Some players are more likely to rise to a comeback when there are only 2 stocks. Some players are more likely to have a comeback when there are 3. Unless you can empirically prove there are more Player C's than Player A's, please stop using the argument.
I've seen enough comebacks happen in a 2 stock environment and I've seen blowouts happen in 3 stock environments. I've also seen enough 2 stock blowouts and 3 stock comebacks to know that we can't just make blanket statements.
Comebacks are comebacks. They will happen. It's not our job to try and make them happen. That's up the players. Great players will rise to the occasion, regardless of stock count or comeback mechanics present. Artificially changing to the rules to try and increase their frequency is not only dishonest, but cheapens the entire concept of a comeback by making it more common and mundane. At no point in hosting a competition should we ever be asking ourselves "Well, how can we make this easier for the losing player." That's up to the game designers who built Smash and players playing it. That is not a decision for the organizers. That is not our job, nor should it be.
tl;dr
Stop using comebacks as an argument for your preferred stock count. I don't care what side of the fence you're on. Stop using that argument.
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My credentials: Competitive in every official Smash game. Head TO of Smash 4 in my region (Ottawa). Met, talked to, and played thousands of different players over the years.