I'm prefacing this by saying I don't own a Nintendo Switch. I am by no means a Nintendo fangirl, I'm just here because I like the Smash franchise. By the time I do get one (for Smash, of course), there will likely be a fair amount of DLC out. Which means that I'm totally indifferent to how much characters there are in the base roster. If it turned out there were only two fighters left, it wouldn't be a big deal to me.
With that out of the way, I find it hard to believe the box theory. Partially because frontloading all the fighters so much feels odd to me, but moreso because they created an entirely new toggle feature just for echoes. I find it hard to believe that Daisy, Dark Samus, Chrom, Ken, Lucina, and Dark Pit are the only characters it was made for - two of which being returning characters who were retroactively made echoes. Especially when we have characters like Raichu, Ms. Pac-Man, and Black Shadow/Blood Falcon who would be easy echo candidates.