Right after the end credits and the archival clips finish rolling, the words "HAPPY BIRTHDAY SONIC!" appear, as well as a screaming crowd yelling those words. You deserve it after all these years, Sonic.
It's more Heartwarming if you know that that crowd are real Sonic Fans all over the world that Sega asked to record in several events to use in this game.
"Hey, Sonic! Enjoy your future, it's going to be great!"
Considering even how his contemporaries celebrated anniversaries that happened around the same time, although many got new games, remakes/re-releases of old classics, and big worldwide events; none of them got a singular game that celebrated the full history of the character in its entirety, good and bad.
Silver's reason for fighting Sonic on any repeat plays of his Rival Battle? Just for Fun.
What adds to this is that if you look closely at the intro on any repeat plays, both Sonic and Silver are smiling at each other.
Right at the end, Classic Sonic attempts to repeat his older self's Boost. Doesn't work right at first, but Modern Sonic applauds and tells him "You're only going to get better!"
Maybe it's just this troper, but when the Time Eater attacks, What do both versions of Sonic do? They immediately rush to put themselves between Tails and danger.
It's even better in the 3DS version. The Sonics are down for the count. The Time Eater tries to finish them off, but the Tailses shield them.
Modern Sonic: Tails! Are you okay?!
Modern Tails: Owww. I think so...
Modern Sonic: Why did you do that?!
Modern Tails: Heh heh.
Classic Tails: I wanted to help, too... Sorry, Sonic.
The end credits showing 20 years of Sonic history in gameplay and music can really warm the heart of a lifelong Sonic fan. It shows that even throughout 20 years, all the Sonic games and their nostalgia is appreciated by both the fans and SEGA.
Sonic's friends cheering both Sonics to stop the Time Eater and both versions of Dr. Eggman before the Final Boss fight.
The sheer amount of dedication put into the music for the game is astounding. Every main track is a remix or redo of the original, and there are many more of them for either mission music (such as a revisit of Sonic R's Super Sonic Racing) to several of the custom music unlocks, such as redos of the Sonic 3 & Knuckles tracks and Sonic Advance tracks (and the US version of Stardust Speedway). The amount of artists brought in to redo their old work must have been a monumental effort, and it pays off in the game. There are many awesome moments when it comes to the music, such as Ted Poley doing a new recording for Escape from the City, to mixing in both Seaside Hill and Ocean Palace into one track, to redoing the US version of Sonic CD's Stardust Speedway track just for the hell of it. Some tracks even come from the really obscure Sonic titles like Sonic R and Knuckles Chaotix.