Actually, Pikachu only lost to the Snivy because it had been overloaded by an electrical attack by Zekrom and was unable to use its electric-type moves.Its far too late to replace Ash. He's been in far too many episodes for the writers to just cut him off from the anime. Its a huge shame too, because if the writers had decided to cut him from the anime earlier, then I believe its overall quality would have been MUCH higher.
The primary reason many of the anime's shortcomings exist in my opinion is because the writers insist to keep Ash as the main character for as long as the pokemon series goes. This is a large issue because it greatly limits Ash's character and forces him to make a huge number of both questionable and stupid decisions throughout the series for newer viewers. Having a new protagonist would at least make these stupid decision understandable, as well as allow a greater diversity of side characters and antagonist to be added into the series.
The writers even had the absolute perfect opportunity to replace Ash with a new protagonist after "Memories made of Bliss", which would have been the perfect conclusion to his story. But instead they still use him as the main protagonist in Best Wishes and not only reveal the shocking truth that he's still ten years old, but also show us that his Pikachu lost to a low level Snivy despite being able to defeat a Latios only a handful of episodes earlier. Ridiculous.
One other thing that I think that pokemon anime could do is to, for the next series, have Ash become a more experienced trainer who meets up and helps out a younger (yes, Ash should actually age) trainer who is new and as inexperienced as Ash was at first.