If I may argue against this, throughout the whole interview they kinda talk about Nintendog a lot. The sentence that you bolded is not meant to say that Sakurai thinks they have no moveset potential, it means to say that there are certain franchises with characters that can't be realized for a fighting game. That's the whole purpose for them bringing up Nintendogs. It is/used to be somewhat popular, but can YOU imagine comeing up with a moveset for a character from that game? This is why there are Assist Trophies like that math one, or the pong game. After doing some research, there's
this article from source gaming made by PushDustIn (that one translator dude) that contains this quote:
"Characters are not picked at random. After fully understanding the character’s personality, development advances while taking into consideration how those characters can be fun to play in “Smash”.
There are some characters that can’t join the battle, even if they meet the qualifications."
Assist trophies aren't meant to be for characters that seemingly have no moveset potential, it's just a means of adding characters that couldn't get into Smash as a playable fighter for one reason or another.