As a professor who has to grade my belief is that word limits are good. Character limits are even better and I’ll be damned if anyone will tell me differently. So when Twitter leaked or something that it was thinking of raising the character limit by more than 7,000% (from 140 characters to 10,000) I was duly outraged. Why, there is no way I am reading more than 140 characters!
And that is the point. First of all, I often read more than 140 characters from a tweet if it links to some longer article. Second of all, Twitter was still going to have the 140 limit on the feed with some other indicator that there is more if you click through. In other words, all Twitter were trying to do is to get more of the content on Twitter itself.
But third, suppose that Twitter were not doing that and the limit was increasing. For almost a decade people have spent their time carefully crafting concise tweets for impact and have developed the hashtag to a level of use that no one had anticipated. If the limit were lifted tomorrow and some people — you know who you are Marc Andreessen — started using more than 140 characters, people would unfollow them. That kind of bandwidth would muck up the reading experience too much. Put simply, social norms now dictate that you keep tweets short and I would guess that it would be terribly hard to dislodge that.
That said, if some people — you know who you are Donald Trump — were not constrained and were to start writing more than 140 characters, maybe just maybe people would stop paying attention.
Let’s face it, Twitter’s problem is that it thinks it has some control over its platform when, as it turns out, how it has evolved and how it is used is now controlled by users and a social equilibria.
Digitopoly 
And then, there is this huge mass of people who either don’t use Twitter at all, or use it to follow maybe just one person. I, for example, use Twitter to follow just Paul Krugman who is otherwise difficult to keep up with. He’s one of those people who post nothing but links to longer articles. OK, I confess that yesterday I added one more that is like Krugman. It probably won’t stay long.
The problem Twitter has is not really that the users are controlling how Twitter is used, but that most people don’t use Twitter at all.
For me, at least, Twitter doesn’t seem to have any particularly useful purpose. I don’t have 10 characters I want to post on Twitter, let along 10,000.
The disadvantage of the current 140 character limit (though I think it’s largely the nature of Twitter rather than an absolute limit) is that Twitter seems primarily suited for sending out easily misunderstood one liners. It’s hard to say “I disagree with this aspect of what [your favorite politician] said in [his/her] latest interview” in 140 characters.
It’s very easy to say “[your favorite politician] is a [racist/socialist/whatever] idiot” in 140 characters.
ThomasW, I think you over estimate the typical poster on Twitter. If Twitter does expand their message size, I expect short insults will continue apace.