I'm not an old player, I only started last year. But the game seems a lot more alive than it did a year ago. Obviously this is from Sinnoh finally releasing, but it's been out for a month and I still log into server queues every once in awhile (usually early in the morning, probably due to Chinese population). Who knows how long this will last, but it definitely shows this game has a lot of life still in it.
I honestly think even if the English playerbase completely died out that the Chinese players would keep it alive and worth the dev's time to keep the game going for years to come.
I do believe we need some real end-game PvE content desperately though. I personally don't mind the current end-game since I'm mostly into casual PvP but from my experience with getting others to try out PokeMMO, they usually quit after they clear all of the story-lines because they're PvE players.