The Evolution of Pokemon

From its initial inception, Pokemon was a world wide phenomenon. The animated series, movies, video games, card games, and endless merchandise have accounted for over 2 decades of Pokemon Evolution. But as the franchise grew older, somethings within the animated show needed to update. How exactly did Pokemon evolve into what it is today? And how did we finally get to a point where Ash and Pikachu are leaving the show after 25 years?