I can't remember where I read it, but there was talk about finishing up the manga after the REBUILD series is completed. Don't know how that will turn out.
" What is it about the series that just makes it easy to replace a long term character like Misty or Dawn for that matter, Misty traveled with Ash for 3 seasons and Two generation. May got 2 and Dawn got 2 but no one every makes it into the next gen why? "
The main focus is Ash, the plot revolves and evolves around him. It's not that the females in the series are expendable, rather they accompany Ash to help the storyline. To get technical, Satoshi Tajiri made Ash as a representation of him, as a kid. Tajiri also stated in an interview, he " felt the character represented the concept of the franchise, the human aspect, and was a necessity." So that's why the girls don't accompany Ash for extended periods of time.