Spring Man wasn't the alternate choice. Ninjara was.
Hey, I missed this the first time around.
So frankly, you do have to do a bit of critical thinking here to see why Spring Man was considered. The reason I say this is because Sakurai himself doesn't actual outright say he was, neither in the presentation nor the Famitsu about Min Min.
At around 6:25 in the presentation, just after Sakutai mentions he was thinking of Min Min and Ninjara, he mentions that most of us fans would likely see Spring Man as the main protagonist, and thus we would assume Spring Man would be the one who would become playable.
Mere seconds later, Sakurai says that Yabuki suggested that all the ARMS fighters are the protagonists or leading characters.
What this suggests when dwelling on it is that Sakurai found it important enough to mention that Spring Man, who we all envision as the main protagonist and thus expected him to be the chosen character for Smash since new franchises have always worked this way in the past, isn't viewed that way by the lead creator of the game and that this justifies Min Min's inclusion in addition to Yabuki wanting her specifically and her being the most popular character voted by fans. This suggests that from Sakurai's perspective, already being a spirit, costume and AT wasn't enough justification to not make them playable in the eyes of most of the Smash fan base and perhaps even in his own eyes.
More importantly, is suggests that Spring Man was likely a contender and possibly even the first choice Sakurai had in mind originally before speaking more in-depth with Yabuki and getting that direction and perspective from him, and from there his request and the general popularity of Min Min made her the best choice since she can be made to faithfully represent the game, please his colleague, and the ARMS fans. Ninjara is clearly Sakurai's favorite ARMS character which is why he was also a possibility.
What is the most interesting thing to note here is that Sakurai never says here at all that Spring Man's AT or any other content of his in the game is why he wasn't selected, which suggests that while it may have ultimately had something to do with the decision by the end of it, it wasn't a heavy deciding factor and most definitely wasn't the one that sealed his fate: it was Yabuki's perspective and the fact that better options came from that perspective.
We also got Spring Man's stage with Min Min as opposed to her stage or any other stage, which I find interesting. There are many reasons for this, I just find it interesting to think about, nothing more.
In conclusion: Even if Sakurai didn't outright state Spring Man was considered, the fact that he thought it was important enough to address why he wasn't chosen and that the reason is because all the characters are main characters instead of just Spring Man and knowing how Smash goes for the main character of new series/franchises, this suggests Spring Man was considered early on. Since Sakurai never mentioned anything about his AT in his explanation, this suggests that his AT wasn't a major factor for his exclusion, and since we know that a PNG didn't stop Min Min this tends to suggest that prior content of characters in general isn't of much consequence to Sakurai and his team when it comes to choosing characters.
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