Concerning player numbers, I have no doubt, they'll continue to rise.
After 5 years, BF2 peaks at around 8000 - 10000 players online per day. BF1942 only did this in its prime.
Thus I predict BF2's longevity will be similar to that of counter-strike's, which means we can't even see the end of the tunnel. BF2 peaked 4-5 years ago around 20k, or maybe 30k daily.
However, our task will be to get BF2 players playing mods instead. If BF2 has a 4000 daily peak in 4 years time, and if we've got eastern, pacific, and more, having a 1/4 chunk of those players wouldn't be unrealistic, though perhaps a bit hopeful. BF2's player numbers may even increase again. The ultimate key to BF2's longevity will be the mods, which are just now starting to mature. Which is why I think BF2s player numbers won't drop anytime soon, perhaps even increase.
Even though new games are coming out, there are no games in sight which threaten BF2's niche. This is why BC2 had minimal impact on BF2 player numbers. Have no fear, FH2 will grow larger for years to come, there will be servers for another decade. In two decades I wouldn't be surprised if it ended up in the history books.
For those saying Frostbite/BF3 is on the horizon to fill BF2's niche, think again. Frostbite requires lightyears worth of optimization or computer technology before it can handle 64 players on a 2x2km map (BC2 only loads a section of a map at a time). Either that or be downscaled to beyond what BF2 is. Eitherway, I wouldn't expect anything soon.
