Think of it this way: without imposition, the act of creation can not happen. Without limitation, everything goes, which funnily enough means that nothing happens, because everything is equally likely to occur. (I know this is very philosophical and meta, but you get my point).

Creativity and expression requires restriction, and every medium restricts automatically. That's why the medium IS the message.

Limitation isn't inherently bad. I also don't think that 140 characters automatically rot your brain for what it's worth. Attention algorithms are a far bigger problem (that's why YouTube can rot your brain too).