You think you're arguing the value of constraints, but you're actually arguing that different media formats have different points of cultural optimization.
If that's true, then is there any need to enforce an artificial length limit on tweets? If shorter, pithier tweets are better, then most tweets will be short and pithy--naturally.
Why are youtube videos mostly 5 to 7 minutes? Why are pop songs 3 minutes long? Why are sitcoms 30 minutes long?
Forms of expression are defined by their constraints. I think of twitter's relationship to prose as analogous to a haiku.