So right here you've pretty much made it clear that you know exactly what the new character would be like, which of course you don't. Again, you're just assuming all of these things and you're assuming the writers wouldn't be able to make the character interesting to anyone because you can't imagine him being interesting to anyone. You're placing your own views on the fans as a whole. And I think the writers have made it clear with BW that they really don't care about continuity and that its only fans who do. Again, you assume that a new character would ruin the "positive image [P]okemon" when there's no proof to suggest this would happen. Ash himself has done more this gen to ruin that image that anything else. Risks are something that have to be taken sometimes. There are times when you just can't be 100% sure on anything, but you have to take the risk so you can avoid stagnation.
What do you consider a few opinions? And ignoring that, I think you're still ignoring the fact that there are a significant number of fans who dislike Ash and want him replaced. Whether its 50%, 30% or 25% that's still a significant portion of the fandom that hates the main character and wants him gone. And that's not considering new fans, who as you and others have pointed out don't know anything of the history of the anime so really wouldn't care one way or the other seeing as they don't have the connection to Ash based on years of adventures.
Read my words again. I said "[Y]ou're implying that most fans watch the show for Ash and nothing more". I made it clear that though you're not coming right out and saying it that's how its coming off when I read your words. And i'd like to see your proof that most fans see Ash as the driving force behind the show. As I said before, I don't think most kids have been around long enough to think of Ash as some necessary part of the series, so they wouldn't have a problem with him being replaced with a new character.
I'm glad you brought up your last point so I can point out its absurdity. Why would every character cease to exist because Ash was no longer the main character? Why would Ash himself cease to exist? There's no reason you should think that those characters would stop existing. They simply wouldn't be main characters anymore and would all gain different roles. As Gym Leaders, Misty and Brock have more reason to appear than Ash (Assuming the new kid was from Kanto). Ash could still be mentioned as they traveled with him for so long and there's no reason he couldn't appear in a mentor type role to the new kid. He could be a member of the Elite 4, a Gym Leader, the Champion, etc. In short, saying that we shouldn't replace Ash because old characters could possibly be seen again is flawed logic, because they can always be seen again no matter who the main character is. These people aren't tied to Ash, they don't exist solely because of him.
It seems like you're assuming a new character means everything that comes before is washed away, when that's not the case at all (Though the writers of course could make that happen). A new series could easily take place in the same world.