It's a shame we don't have yellow gremlins anymore to give rather accurate census of the active player base.
As this is mostly disucssion on how it feels. I don't remember Ishtar ever being more populated than stygia though but i stop playing a few months before the merge. But you're right it didn't merge into stygia.
Anyway given how for several years, you had to raid to get the best weapons in the game or gear ... to PvP. That trend doesn't seem abnormal. I'd be particurlarly interested in the stats six months to a year after khitai.