Let's look at the facts. Ash is a loved mascot and has been associated with pikachu for years. They used the movie 20 logo. Pikachu has had lots of character development so why let that go down the drain for another one?
It's just wishful thinking on your part.
Edit : If they wanted to replace...
You all hate on Ash but he's done so much for his friends and Pokemon and the world. Heck he's been acknowledged by many important characters. Ash is special to me and that won't change. Besides Pilachu needs a reason to stick around.