You know that is an argument about expansions in the first place? Significant player dropoffs in WoW happen in the last patches before new expansions. The userbase usually spikes up on new releases, but almost always drops lower every time, predominately because new releases gate players who were on old versions against those on the new one. Friends stop being interested, unsub, and never come back because the player base is fragmented across every level cap WoW has had.

I've always thought that level caps in MMOs, particularly the WoW variety slash-and-burn where you basically make a new theme park every two years and throw the last one in the dumpster in terms of player viability, is a destructive cycle not conductive to building real living worlds.