Fair enough. I get where you're coming from, but I still cannot see as many drawbacks to this as there are benefits. I understand you're in the boat that this is a slippery slope, and it comes down to "Well, if this goes through, where is the limit? Where do we say enough is enough?" Although I personally believe enough is enough when one input becomes more than one action, if you understand what I'm saying.
To be honest, I feel a lot of this is going to come down to comfort in the end. Despite the massive benefits that the Smashbox could provide, do you think most players are going to give up their comfort zone to basically re-learn how to play?
I honestly don't think most will unless they were really just uncomfortable with the GC controller after all and only used it because it was the only option (like me, to be honest). This sort of comfort zone stays even on a micro scale, and especially a major one. (Example: Almost nobody uses Bidou control scheme in Smash 4 despite it allowing you to do so many things and gain a serious edge because they'd fall out of their comfort control scheme and have to pretty much re-learn how they play the game). I can only imagine it will be far worse when learning an entirely new controller.
The slippery slope is a real issue, but I think even as-is, the Smash Box is already bannable. You are against 1 input resulting in more than 1 action, but shouldn't the difficulty of the inputs also be comparable? How do you even quantify an analog input like a shield drop? Moving the stick into the shield drop zone almost always takes more than one frame on a GCC, so as far as the game is concerned, it takes multiple inputs to shield drop. The Smash Box does shield drops with just a single input in a single frame, and by this logic it could be considered a macro. It is performing an action with 1 input that normally requires 4-6 depending on the precision of the player. This is the fundamental issue of modded controllers. As soon as you allow someone to decide how to perform inputs, they are going to choose the easiest method. If I put a volume dial on my controller, does spinning it 1 degree count as a single input? If so, I can easily get insane down-B mashes with the Mario brothers by spinning a dial. Buttons aren't the only way to perform inputs, which is why allowing players to decide how the do inputs will result in unfair controllers.
I'm not sure what massive benefits you're talking about are. I'm assuming you mean ergonomic benefits, but it's already been explained that the vast majority of players can use a GCC properly and totally avoid hand issues. Even with all the players we have not playing properly (i.e. not taking breaks, stretching, or learning to perform inputs delicately), only a very small percentage have had significant health consequences. It sucks that some people can't compete because they have physical limitations, but this is true in all sports. If you have bad knees and want to play basketball, you can't design some new kind of shoe with springs that allows you to jump higher with less effort. It doesn't matter if you are jumping the same height as every other player on the court, the fact that you're doing it because of your shoes is unfair to other competitors. Melee and basketball competitions alike are designed to test physical skills, as well as mental ones. If your body won't let you perform the necessary physical actions, then you can't compete with other players on an even playing field.
Smash 4 doesn't seem to provide massive benefits to players that are more technical, but Melee clearly does. You get massive benefits by replacing all of your wavedashes with perfect angle wavedashes, never missing recovery angles, and worst of all, being able to do ridiculous multi-SDI. Idk why people think it would be so difficult to learn a new controller. Sure it will be awkward at first, but Hax has already demonstrated reasonable competence with the B0XX. Part of this is due to the fact that, like I said before, the difficulty of the inputs is largely being negated. Doing shield drops or perfect wavedashes on a GCC takes years of practice to hit, and even then players still miss sometimes. On a modded controller, these are braindead button combinations that require no level of precision, all you have to do is hold the right button and you never mess up. Someone who's never played before can practice shield dropping on a Smash Box for 30 minutes and achieve a higher success rate with perfect shield drop speeds than Plup on a GCC. If someone doesn't see an issue with this, idk what to tell them.