Im pretty sure that's the reasoning people use for a ton of 3rd parties. However, they don't compare to what we've gotten.
Something I thought about earlier...but think about the Top 5 Multi-platform 3rd party characters with a definitive legacy in gaming that are left aside from who we have.
Snake- iconic character from series that revolutionized the stealth genre. Elements of Metal Gear gameplay have left a footprint on other games of the genre, and the series is referenced throughout other games (actually, there's a reference to it in the new Batman game!) Has plenty of popularity in the Smash Ballot.
Rayman- another platforming icon. May not be up there like the ones we currently have, but he's a fairly well known character to gamers and non-gamers alike. Seems like a fairly popular choice in the ballot
Uh...
Simon Belmont- Castlevania is a staple of retro gaming, and is a series that has lasting gameplay impressions on other games in the industry. I don't know much else about Castlevania (never played it), but i'm sure somebody else could go more in depth.
I don't know guys. 3rd parties aren't just like any old 1st party character, and I don't know why someone would think they would be. Every 3rd party we've gotten has been a pretty substantial inclusion. Plenty of 3rd parties simply do not stack up with who we have.
To be fair, Snake wouldn't have been in in the first place if he wasn't a gaming icon. Sakurai has said his piece on adding 3rd parties, and if Snake was say for example, Super Meat Boy, I would have to think things would be different.
Interesting note on 3rd parties: Each of them premiered during earlier gaming years...Also I just realized I didn't list 5. I couldn't even come up with anybody else! (Disclaimer: There are plenty I like, but not many that I feel have the starpower of the featured 3rd parties we've already gotten)