I think Nintendo characters fall under two categories:
- The charismatic type: These characters are cute, colorful, funny and very memorable. Even if they don't say much they shine thanks to their happy attitude, colorful dispositions and cartoony attributes.
Examples: The entire Mario cast, Pokemon, Kirby
- The enigmatic type: These characters shine thanks to their mysterious, dark stories. They don't say much and keep a lot of things secret, making the player yearn to learn more about them, seek out their past and guide them through their quest.
Examples: Link, Samus
Nintendo is the type of company that doesn't care for trends, what's popular and fads. They aim to create fun games everyone can enjoy, they love going against a train of thought and their characters reflect that mentality.
While many characters have either gone dark (Jak and Daxter) or gotten cooler (Sonic) Mario is still Mario. Yes, he now speaks, laughs, hoots and screams in a high voice, but he is still the charismatic, timeless plumber we met in the 80s. Link may have gotten several makeovers but he is still the quiet hero that has skills and courage. And finally, Kirby is still cute after all these years.
Like the GO mentions, it also helps that their original creators are still working at Nintendo and are still supervising the latest games, even the spin offs. Sonic' creators are everywhere now, and none of the best designers stayed behind.
I think I can guess what the GO will suggest on the next episode; For Sega to try and make nice with the original Sonic Team members, to focus more on basic gameplay and less gimmicks and give Sonic a solid personality rather than just making him the cool, "Poochy" like hero he's been since the 90s. Create a timeless personality so to speak.