This argument misses on one major theme: future growth.
Looking beyond the survey’s bias towards active users and beyond the potential of a competitor offering a similar service for free, Darren’s view is nearsighted. It is only worrying about current users of Twitter.
Yes, it’s possible active users of twitter who place a value on it are willing to pay to use its service each year. But a user who has never used it? No chance. The freemium model if anything would be necessary to gain initial adoption and trial, especially in new countries. And any strategy for Twitter that kills growth is an absolute no-go.