The origins of the 140 character limit are interesting; it was originally put in place because Twitter was SMS-based, and SMS limits the number of characters to 160. 140 allowed some wiggle room.
But retaining the character limit changes the platform entirely. "Lifting the twitter limit" does not a publishing platform make; it changes everything that Twitter is.
I think Jack explained the psychological reasons very well [1]: Imagine you're standing in front of a blank wall, and instructed to paint the entire thing. You hold yourself in a very specific way. You spend a lot of time and effort thinking about how you should do it. You compose your actions in such a way to fill the entire wall.
Now compare that to being given a post-it note. Not only is it less intimidating and more disposable, but it doesn't take as much mental effort to get to a point where your work becomes valuable.
That's why I tweet incessantly, but I rarely blog. To blog I feel like I have to have some continuation of several thoughts that are strung together in a logical way, and all flow to some larger conclusion. Tweeting is a single thought or impression that is off the cuff. It almost flows naturally out of me.
I even find that when I want to write a blog post, it flows the best when I just start out tweeting. Before I know it it becomes a tweet storm, but breaking the work up into very small, bite-sized chunks makes the entire process manageable.
That being said, it makes sense for there to be some way to easily consume a tweet storm other than someone tweeting 1/ 2/ 3/ 4/. But I don't think that solution is saying, "Alright, go nuts, tweets can be as long as you want them to." My hunch is Twitter would lose an order of magnitude more value than it would gain by doing so.
[1] https://www.youtube.com/watch?v=EPLK8lmsjq4