I want Isaac in Smash because I loved playing the Golden Sun games back when I was younger. Although, for King K. Rool, I never had the chance to play his games but with the DKC Trilogy and DK64 on the Wii U VC, I can now. Looking at the OP now, I do believe it does need a little bit of an update.
Most of the new characters added to Smash debuted in recent years therefore, they're more relevant, i.e. Shulk, Lucina, Robin, Wii Fit Trainer, Rosalina & Luma, Greninja, Dark Pit, and an alternate, Alph. Little Mac reappeared not too long ago in the Punch-Out! remake thus making him a non-retro character, same for Palutena with Uprising. The BJ alternates, Koopalings, have been appearing in recent Mario games. Relevance certainly is still a factor in choosing characters in Sakurai's mind.
If anything, Wii Fit Trainer would've been extremely unlikely as a Smash fighter because of no moveset potential but we now see what Sakurai did with that.