I'd say that given the part that keeps the most people around is the launch glut, Blizzard should focus on frequent content updates and/or finding a way to make the leveling experience more replayable.
I played all twelve classes throughout Legion. Did not mind leveling them at all because the artifact acquisition quests and the class campaign added enough variety to the experience. Many CRPGs have people doing multiple playthroughs because through choices (story or group composition) the gameplay in each playthrough is sufficiently different. Blizzard has tried that with Chromie Time and now in Shadowlands by letting you level in different ways; I think they need to dig deeper and iterate on this.