As for Ash being replaced by a new main character, i dont see how would that make things more fresh in reality when his replacement would still do gym quest, have Pikachu as main mascot and do things which Ash already does. That would be rather pointless.
Not to mention Ash can be refreshed as character too. Through new pokemon, clothes, new subplots being introduced within story etc. Its not like older character cant bring new and unexpected twists to story too, being another reason why i dont see any point in having Ash replaced. Granted his friends do get replaced(for rather shallow reasons going by mr. Hidaka interview) but difference is that they arent driving force of this show being its main mascot. Something which Ash happens to be with whole pokemon revolving around his journey