If you stop to think of it, most of the older cast has been gone for years now, and aside from stuff like flashbacks or in the Japanese openings/endings, they simply don't appear in episodes anymore. For example:
- Misty hasn't appeared in an episode in 10 years. Her last appearance was back in 2005 when Ash came home from the Hoenn league. Since Battle Frontier started, Misty never appeared in the anime ever again. Her last episode was AG134, "A real Cleffa-Hanger" and then afterwards she was basically neglected to flashbacks and that's it. Its pretty notable that Misty hasn't had any appearance since AG, which is now three generations and 10 years ago.
- Tracey hasn't had a speaking role since the final episode of AG back in 2006. We've never seen him again since.
- Gary hasn't appeared since the Team Galactic finale back in 2009. Since then he's never appeared again.
- May's last appearance was the Wallace Cup back in 2008. Even with the release of ORAS it seems it wasn't enough to get her another appearance.
- Dawn's cameo in BW back in 2012 looks to be the end of her character forever too, especially with Serena designed to be so similar to her.
- Brock is of course gone for good, having stayed gone for two gens with Cilan and Clemont replacing him. His last appearance was in a Cilan special that only aired in Japan
So given the huge gaps in time for some of the older characters, especially ones without an episode appearance in an entire decade like Misty and Tracey, is it safe to say the writers are done with the entire older cast?