The history you mention is a very minuscule one. SMPRG is just one game among the plenty of critically acclaimed Mario games, and one that isn't remembered all that much, unfortunately.
Think of a person who got into gaming in the Wii era, which arguably make up a large portion of the current Nintendo fans. Will he recognise Geno? Most likely not.
Will he recognise Cloud? There's a pretty good chance he will - Square Enix has made sure to promote him, there's a remake on the horizon and his game was a killer app. Same for others.
For the sake of argument, let's say Geno doesn't need to be of the level of Sonic or Pac-Man, as he originated in a Mario game. Being a Mario character, he should be able to attract a good number of Mario fans, right? Except that most won't recognise him (again). Your average Mario gamer will readily recognise Rosalina, Dr. Mario and Bowser Jr. though, as Nintendo promotes them and they continue to play important roles in current Mario games.
Like it or not, Geno's appeal is rather limited - he will only attract people who have played SMRPG, and they form a very small subset of the current Mario fanbase, and mostly older gamers, who will likely move on from gaming in a few years.
