BluePikmin11
Akko is my dear daughter!
I do not think Sakurai's perception of the original 8 affects anything, as that was long before third-parties would be included in Smash.Honestly, the biggest problem with the supposed "third-parties must be icons" rule is that Sakurai's perception of how iconic or significant a character is may not match that of the fans. Sakurai always personally felt that the Original 8 of Smash were all the greatest Nintendo characters people would think of back during Smash 64, even though I know some people would disagree on Fox. Sakurai also felt Bayonetta was pretty significant, so it's not a stretch to think that Professor Layton could be included because of how significant Sakurai may feel he is. Professor Layton was a defining third-party franchise of the DS and 3DS era with 15 million copies sold as of 2013, which is impressive for a game that was 6 years old at the time the linked article was published.
However that Bayonetta article just blew my mind. I wish I had this article before she was confirmed as the final DLC character. This would have been a very helpful article back then, but it was translated months late.
If Sakurai does pay attention on gaming history, then he would definitely know of Layton's history and probably keeps track of it. Especially noting that at one point, Sakurai and Hino were together at a 2009 conference at the time the biggest Layton game Unwound Future came out in 2008 in Japan.