When Nexon hosted DFO, you could tell it was going to die months before it was going to shutdown, but it was blatantly obvious when the update posters were done in MS paint and they left in halloween dungeons or something like that and overall poor decisions on how to handle the community. DFOG is now being hosted under the developers of the game which is Neople, and it's alive and well.
other games I played were Lost Saga and Rumble fighter, these two games got passed around publishers like it was hot potato( not hot anymore because its just p2w garbage)
and then the last game I played that closed was called Hero Warz hosted by KOG, which hosts Elsword as well. Hero Warz NA was a big fk up, they strayed from the Korean design to make it more unique for NA, however all this created was a boring end-game and really reduced the potential of what the game could've been in North America. that said I played the Korean version to compare, and the KR version was just miles and miles better, it made the NA version look like a really stupid attempt at changing something that didn't need any changing.
the player population for these games were probably less than 5000, that's probably being generous since theyre all very niche games. and midway through their services they'd most likely average couple hundred active players.
the publishers usually announce a end of service for these games, maybe a month before closing and then they have all these events and freebies to let the players enjoy the game before it finally closes.
i think im pretty good at playing games that close down, all wasted potential anyway.