Twitter is in trouble because a website with more features (photo albums, apps, blah blah) and a larger community has been consistently adding all of twitter's 'killer' features to their own site.
Question: What's to stop facebook from adding public status updates or any other distinct twitter feature?
Answer: nothing.
And blogs had 'more features' than Twitter too, but as they have proven, artificial constraints can make something popular. The minimalism is pretty much the point.