Twitter was a bold as an experiment for democratic debate
I disagree. Twitter was originally intended as a "micro-blogging" platform that ran over SMS with a web interface. The 140 character limit is from that, and it's the reason Twitter usernames are limited to 15 characters. (The remaining five characters in a 160-character tweet are for "command" codes. The SMS interface still exists.)
It rapidly grew to something that no longer fits that, but it was never an "experiment for democratic debate," it was if anything an experiment in running a social network over SMS. Public forums and public blogging platforms existed well before Twitter did.
But for posting selfies, spamming, professional self-promotion and reinforcing your own world view, yeah, it's great, and it's true of Facebook, Instagram, everything else.
While true, it's also useful for one other thing: posting notifications. Most of what I use Twitter for are getting notifications about various things. Twitter has essentially become a "push RSS" service where instead of polling an RSS feed, you get push notifications as updates as posted. It's useful in a very specific set of circumstances.
It's that last thing that would make me miss Twitter were it to go away. 99.9% of Twitter? Burn it with fire. But using it as a free notification platform with an SMS-fallback? That's incredibly useful.