Here are some points you did NOT cover:

1. Businesses are currently creaming over Twitter. They use it as a way to humanize themselves, market research (as more people begin to use it) and as a customer service platform. More businesses are doing this and will continue to do so. Consumers expect businesses to be transparent and Twitter helps them achieve this.

2. A platform like Twitter is a way for people to position themselves strategically as experts.

3. You may find Twitter invaluable, but tell that to all the developers who have built 3rd party apps for Twitter. People have built a huge ecosystem around Twitter and that is what makes it valuable.

4. Different groups of people adopt Twitter differently. The fact that it is so simple with an open API, allows people to do what they want with it.

The new Facebook changes are ingenious. They are evolving as a social networking platform to suit the needs and desires of people as well as keep themselves from becoming irrelevant. If Facebook continues to do this, they will continue to reign the social web. It’s really an arms race. Friendster did not do this, thus it is forgotten while Myspace is completely irrelevant.

Twitter will have to make more changes before it can truly sustain itself, but for now, businesses, early adopters, celebrities and people who are confused by Facebook’s new interface will jump onto Twitter.
You made some good points but your post assumes that Twitter won’t change for the masses. Who knows? It might or it might not. But for now, people seem to not care. Like I said before, all social networks have a lifespan. Their lifespans are extended based on whether or not they can evolve and I think we agree on this.