I would be welcome to watching have further development to further reach their goal, however, I wouldn't want that to last longer than it needs to. Seeing one character stay for multiple sagas would eventually become boring. I'd rather a character reach their full potential during their saga, therefore preventing them from needing any further expansion. I mean, were we supposed to continue to watch May or Dawn until they won a Grand Festival before moving on to a new character? And waiting for Iris
to become a dragon master would obviously take forever with her methods of training.
I do agree with you that constantly replacing old characters does get annoying, especially when none of them, besides Brock maybe, ever had their story finalized. However, I personally become bored quite easily with watching characters do the exact same thing for multiple years. Even with new subplots it would eventually get annoying imo.