Of course that is an actual game design reason. If the execution is hard, people can’t just bust out whatever move they want at the drop of a hat. The execution is in itself a balancing of moves. Raging Storm is a ridiculous input because the move is ridiculous, if it was a double fireball motion it would probably be the most broken anti air in whatever game it was in. Same reasoning for the 720 motion, an instant throw that does 50% damage is not something you can have an easy input for, if people can do it whenever they want for no effort it would get to a point where if you are ever within a certain range of Zangief and you aren’t jumping, backdashing, or doing something invincible, which are all incredibly risky options, you’re getting thrown, and that’s just bad game design.
You can say “Well then just don’t make moves broken,” but when everything has an easy input and no move is broken, you get serious homogenization like, and dear god I hope no one takes offense to this, Smash Bros., where all the moves kind of have a samey feel and the only difference between one move or another is how far it hits someone away, how fast it is, and whether or not the character has a sword. THAT kind of design is also bad, but in a different way, because it makes your gameplay stale and boring. Smash has managed to get away with this however by the simple concept of the game and the characters involved, also the competitive scene creating a deep meta that keeps the game fresh on a mental level.
Point being, complex and difficult input has its place in fighting games. Not that ALL games should have it, but there are legitimate game design reasons to have strict inputs. With large input buffers, you give people large windows to do a wide array of things. In KoFXIII for example, because the buffer window is so long that Kim can Max Cancel his flash kick by using an input “shortcut.” I use quotations because there’s nothing short about it. But basically, you buffer the first half of the Neo Max as you’re doing the flash kick, and then finish the input after the flash kick. If the buffer window wasn’t long enough to store Kim’s input for the Neo Max, you would have to do the full legit input for that combo to work, which is ALMOST humanly impossible, and it would drastically hinder his damage options because a good majority of people would not be able to use that combo.
It really just depends on what you’re trying to do from a design standpoint. Should Kim, one of the best offensive characters with some of the best normals and quite possibly the most hard knockdown/safejump setups in XIII ALSO have some of the highest damage in the game, being able to kill a character for 3 meter? Well XIII is a high damage game, and SNK was going for that, so that was their design choice. Someone else on the other hand might have a problem with that and might just make the buffer window a lot smaller so things like that were harder to do if possible at all.
Anyways, just my two cents on the issue.