Pikachu was the main starter Pokemon in Pokemon Yellow right since Bulbasaur, Charmander, & Squirtle were unavailable at the beginning of the game. However, I'm having this strangest feeling that if Pikachu wasn't there to be captured by Professor Oak and the only choice for a starter Pokemon which is Eevee that the protagonist's rival will get anyway before the player, then will the game wouldn't be the same and result to a game over or will be something different and may alter the story a little bit as far as I can tell. In fact, if it wasn't for Pikachu, then the player wouldn't make it that far in the game, right. After all, Pikachu is the main purpose for Pokemon Yellow, otherwise, it wouldn't be nothing else to it.