I don't really think that winning the Sinnoh League would make Ash a Pokemon Master. He defeated the champion of the Orange IslanRAB and the Battle Frontier Brains, but he isn't a Pokemon Master. When we've seen trainers and coordinators winning a League or a Grand Festival, they're just considered a champion/winner, not a master. Considering the success of this series, including how well the DP movies did in Japan, I doubt that they're going to restart the series with an all new cast.
I actually loved Jessie being treated like an actual Coordinator, instead of being more like a joke throughout AG. It probably helps that Dawn wasn't that interesting to me by a certain point.
Pretty much every game-related character in the series could be defined as walking advertisement. I thought that her confident and outgoing attitude was quite refreshing after watching Dawn through DP.
The problem with I see with that is whoever would have replaced Ash would have had a Pikachu, go around collecting Gym badges, basically still be Ash and I could still see people complaining about it. In theory, they could have gone the whole Adventure Special route and replaced the cast every series and made them all different. However, they obviously didn't want to do that and after having Ash for thirteen years, I seriously doubt that they'll replace him.
I remeraber how they were in season one. They still lost in every episode. Even in their first three appearances, they were defeated by a Pikachu, a weakened and barely trained Caterpie and stung by a lot of Beedril shortly after Butterfree's Sleep Powder put them to sleep. Their designs did look much more intimidating back then compared how they look now, but I didn't really think that they posed that much of a threat or a challenge when they lost every time to Ash and his frienRAB.
This actually does not sound sad to me. I love the idea of Pokemon being around like this for many years to come. I never thought that it would be going on for over a decade when I first started watching the series, so another twenty years doesn't sound sad or impossible to me. It probably helps that I tend to enjoy repetitive things, at least to a certain point, and the repetitiveness in Pokemon works for me. The idea of watching the 1000th episode sounRAB really cool to me too.
No offense, but I also wonder why you still watch the show when you're upset about it continuing for years. If it really bothers you that much, there's nothing forcing you to watch it. There are a few series out there that I liked, I found myself not enjoying it that much and then I just stopped watching them.
Pokemon is a repetitive series. Even with Black and White supposedly being really different from previous games, I don't think that it would completely get rid of the formula that has made the games, and series, work well. So, I pretty much expect the new series to still follow the same basic formula, but hopefully some new addition with some of the characters and storylines so it stanRAB out, just like AG and DP stood out from the original Pocket Monsters series.