as a former employee who had the opportunity to sit across the table from some of the execs you read about in the newspapers, let me give you my admittedly skewed perspective.
cop catches thieves.But it is not in the interest of cops if they catch ALL thieves. This is not a vaccination program, where if you eliminate polio across the nation, doctors will rejoice because they can now focus on malaria. If there are no thieves at all, the funding for cops will simply drop drastically & cops will be laid off.
So cops must catch thieves, and always "appear" to catch even more, but never actually catch all of them.
If you had a 100% sanitized feed with no trolls and no spam and no abuse, it would become a plain old communication utility, like a letters to the newspaper column, or weather channel or stock ticker driven finance channel etc. Even in those mediums, the channel programmers throw in sordid details like "most disastrous tornado", "worst all-time hurricane", "sleaziest hedge fund manager" etc. They do that because otherwise it gets too boring and viewership drops.
it is very much in the interest of twitter to "appear" to be actively engaged in combating trolls and abuse. but part of its newsworthiness and charm is all this frenzied action around policing abuse and suspending high profile trolls. these things keep the company hip and happening. if you solve the problem for good, there wouldn't be much to cheer about - users may temporarily cheer, but then they'd notice nothing much happening, no controversy, no abuse, no trolls, everything too polite, and soon users will switch to the next racy medium.