You mean like /b/ was, by the end, supposed to act as a buffer on 4chan to keep out the new posters from the other boards while they were still adjusting to the local "etiquette"? That worked for a while, and then when that board became so bad that even new users wouldn't find any enjoyment in posting in it, they simply stopped lurking and carried over their bad habits to other boards. It spread through the whole site at an alarming rate. You can't try to separate the good content from the bad, or at least not for a long time because the bad always finds ways to spread, and I doubt the separation into "sub-reddits" instead of boards has any meaning in this case.