If they were just going to keep the same character to do Contests for the 5th gen, then May would have never left. The only reason May was written out was because she was an R/S girl, and the D/P girl advertising Contests in her place is far more marketable.
Now that the writers got rid of May despite her popularity, the popularity of Drew/Harley, and the fact that her character was still entertaining, what makes you think they'll keep Hikari next gen?
The thing is that I would rather they just keep Hikari the next time around instead of just replacing her with another girl performing the same role. Of course, I'm pretty sure that Hikari is just as expendable as May was.
On that note, wouldn't it have been marketable to have also replaced Brock, a character from Red/Blue, with a D/P character? R/S/E is pretty old, sure, but not the way R/B is. Plus, Brock's fanbase is significantly smaller, especially when compared to May's (even in Japan). The writers could have easily have had Ash start on a completely clean slate and advertise the D/P games even better if they had him travel with two brand new characters. Instead, they just keep the stale character and replace May just because the game she is based on is supposedly 'old news'. Tch.
If Hikari had another Pokemon related job, then it would've sat better with me. I liked May very much but I would have loved to have seen another female's adventure as well... if it were original. Yes, Hikari does have some different traits (ex: having always wanted to be a Coordinator, a female rival, etc.), but it doesn't change the fact that she has the same goal as May. I'm hoping that her journey won't go down the same, exact route that her predecessor did...but knowing the way the Pokemon anime works, it probably will. I'm still holding out a little hope that it won't.
Now, before anyone accuses me of being some rabid May fan that's just bitter that she was taken off the show, let me just say that I also wouldn't have liked it if Hikari's goal was to be a Water Pokemon master (like Misty) or wanted to be a Pokemon Watcher (like Tracey). It's the fact that she's slipped into the role of a pre-existing character that's annoying.