Ye, it is true what you say, but so far there arent really any indications that shows that you cant play Killzone 2 online, at least in my opinion. I mean, sure, the weekly ranking stats have been gone from each week since Killzone 2 first launched, and this could be an indication that many people are quitting on playing Killzone 2, but there are always easy to find servers to play on, so there is no problem to play Killzone 2 online, at least after my experience. I would say that it is first when there will be relatively difficult to find servers to play on that there are any indications that the KZ2 community might die.
And even if the weekly ranking stats have gone down for week after week, it is not like the numbers of weekly players are at around 5,000 or something, it were about 240,000 players last week.
I just tried to play Resistance 1 online just to see if people were still playing it. I chose ranked games, then free-for-all games (deathmatch), and the auto matching started. It didnt really take long before i was matched up with other players to play with.
If a nearly 2.5 year old FPS game like Resistance 1 still have players who play online, then i dont think anyone need to worry about that it will be difficulty to play Killzone 2 online anytime soon