...I think that they keep replacing older characters with newer ones with the sake of just promoting or seeing homage to new games or old games.
I mean look:
Roy was used to promote for Fire Emblem 6.
Replace by Ike to promote for PoR and RD.
Young Link was used as a tribute to Orcarnia of Time.
Replaced by Toon Link as a tribute to Wind Waker.
Mewtwo, a character from the 1st generation Pokemon games.
Replaced by Lucario, a character from the 4th gen, also gave homage to Pokemon D/P
See where I'm going? Marth will return. Sadly, Ike shall be replaced. (NO!)
(Pichu = A lighter, faster, and more emo version of Pikachu. I mean, wtf, who wants a pokemon that hurts himself 2% everytime he attacks?)