Explain how Lucario would be nerfed?
For Lucario to rubber band, he has to inherently be behind that many stocks, and the smaller the stock gap gets the less strong he will be anyways. If he kills the first guy's stock while he's on his last, he's still on his last stock and his power goes down from killing the other guy anyway.
1. Lucario in 3 stock scenario at 100% damage on his last stock. He kills the other guy for the first time.
2. Lucario in 2 stock scenario at 100% damage on his last stock. He will be weaker than the first Lucario, but his opponent also doesn't have 3 stocks to begin with. He is at the exact same power level as if the guy had already lost a stock and Lucario was trying to take the next one. It may be harder for Lucario to take off the first stock here, but that's because the person inherently is closer to losing all his stocks since he's got 2 instead of 3. Lucario's power level therefore is the same once you get to the same stock margins. And before that, even if he's super powerful from 2 behind, he loses this power and it resets to 1 behind once he kills the other guy.
Shortening stock count would only nerf Lucario if Lucario's power permanently grew based on stock. If you lost your first 2 stock, and retained this power for the REST of the game, yes switching to 2 stock would nerf him. But the game already balances out his power based on the stock margin between players that no matter what count you start at you eventually reach the same power level for killing the other guy.