Realistically speaking,there is no chance that Ash will get replaced with Dawn.Whole point of this show is story about Ash and his travelling to become pokemon master,and im surprised to see from where people are getting at that his could happen?
When Ash leaves,that will be the day when show ends.
Dawn will most likely ended up being replaced like it was case with previous two females after next Battle Frontier,with yet another girl coordinato repeating cycle all over again(sigh).Talking about originality.
Ash and Pikachu are icons of this show being most popular characters,and replacing them after 11 years well wont polute very good results leaving positive impression on fanbase.Which automatically pull with itself drop in tv ratings.
For the record,Brocks leaving shaked up this anime,Mistys leaving was even worse lefting big mark on it causing people to stop watching this show after said incident(not the only reason,but one of them for sure),and i dont need to even explain what would happen if Ash leaves and Dawn stays.
Personally i almost stopped watching this show when Misty left,and if that happens to Ash aswell i dont see any point in following this anime anymore.