Respectfully, I disagree. The "SMS isn't important" argument is why Pownce failed. 50% of twitter's content is sent in via SMS. It's not consumed via SMS so much, but it sure as hell is produced that way.
And remember that the mobile Internet in the US is pretty good. In much of the rest of the world (the places where people have mobile phones but don't have computers) mobile Internet is still slow and poky. The switch-over from SMS to Internet hasn't happened even in the US yet, so it will be a long while before it becomes a good idea for Twitter to drop SMS, on the order of 5-10 years.
More people want the option of receiving Twitter by SMS if they have to than actually do by choice day-to-day. If being out of good signal meant not being able to use the service at all, people would be less keen on incorporating it into their lives.
And remember that the mobile Internet in the US is pretty good. In much of the rest of the world (the places where people have mobile phones but don't have computers) mobile Internet is still slow and poky. The switch-over from SMS to Internet hasn't happened even in the US yet, so it will be a long while before it becomes a good idea for Twitter to drop SMS, on the order of 5-10 years.