If Sakurai actually said this through an interview, I think it would get a lot of people pissed. :L
TBH, I am not sold on this possibility, there is still something about this kind of decision that is so un-Sakurai like to me.
I cannot put my finger on it.
I think a good example of the idea would be Bowser Jr. in Smash 4.
Sakurai openly stated in a Nintendo Dream interview that Bowser Jr. was on the brink of being cut, but just barely made it. Now, look at the scenario if he was dropped because the staff didn't think they could finish him in time? Given Sakurai's DLC philosophy, I don't think he would've saved Bowser Jr. for DLC. Rather, I think he would've taken what Bowser Jr. already had and worked him into an Assist Trophy. Like I said, ensuring the character is in the game in some capacity, and preventing the effort put into developing him from going to waste.
Now, although that scenario is different than the one I originally made, both could apply to playable characters being made into Assist Trophies. Either opting to turn partially developed characters into Assist Trophies
in favor of clones, or maybe the development team just felt that they couldn't finish those characters in time, and thus made into Assist Trophies.
Either way, I think its very possible some of our Assist Trophies were on the shortlist of unique newcomers at some point.
I actually can't see the ATs we have as playable characters. Maybe Bomberman/Shovel Knight but wouldn't they negotiate the character's role with the third parties in the first place?
I can definitely see a few Assist Trophies, old, new, Nintendo-owned, and 3rd Party, as playable.
With 3rd Party characters, its possible that they would have their role defined while getting their rights, but there's no guarantee that is the case. Things change during development, and I think it's possible Sakurai would've given himself some 'wiggle-room' to change things up if, for whatever reason, a specific 3rd Party character being in a specific role was no-longer realizable.