You haven't looked very hard, have you.
Here are the activity levels for WotLK:
The URL as text:
https://web.archive.org/web/20170113...onactivity.php
The only activity drop is around New Year holidays.
Compare to WoD / Legion which had HUGE drops and bumps.
I repeat, the "cyclical" behavior only appeared in material amounts since WoD. It started being noticeable in MoP in a much tamer form, but it only got serious / activity-defining in WoD (and continued to be serious in Legion).
Prior to WoD people had enough trust in Blizzard to stay subbed through rough times / had enough to do during rough times / rough times weren't so rough, you can mix and match these things as you wish.