Wrong. Twitter not only continues to thrive, its competitors continue to flounder. Check it out:
It's actually even better than that for Twitter. Avid Twitterers use the continued down time to not just complain, but also to tell everyone else how unbelievably great and valuable the twitter service is. Every other start-up wishes it could be so lucky.
Now there is the risk that Twitter could become the next Friendster and alienate its user base so badly that they end up fleeing to another site. The network effect has strength, but it does tap out. People are already commenting that they'd leave Twitter but for the fact that their friends are still on there.
So what would make people leave Twitter? Who knows? But it's clear that it'll take more than just better architecture. That problem actually is growing their audience.