I think you nailed it. Number of followers is the ultimate vanity metric. Looks important on the surface, doesn’t actually mean much.
If you look at newer social networks like Snapchat, followers are displayed at all. But Twitter was born during an era obsessed with public ranking on social networks — think of the “Top 8 friends” feature on MySpace.
Earlier this year, I predicted that Twitter would start to downplay its follower counts. I’m still waiting.