This isn't about Kasumi and Takeshi, it's about Satoshi. Why should he be replaced? It'd be the exact same thing Pikachu, Badges, League Lose, Gym Battles, traveling companions. Rinse and Repeat. What's the point and why take the risk?The writers can drop Ash for the same reason any other main character has been dropped. And yes, Ash's friends aren't as important as he was, but they were still once in every episode and in the early days, Misty/Brock "seemed" as important to the anime in general, and now they're both gone.