I wonder how much of root of this can be reduced to election year political nonsense. A year from now, it's going to be much less annoying to be on the Internet.
The author is right that the product has some major flaws, especially around harassment, but no one else seems to want to take over the space. And Facebook, Tumblr, G+, etc. are all much worse platforms for the kind of discussion that happens on Twitter.
So I think the toxic environment can definitely put a ceiling on the number of new users, so does the timeline issues, but it could probably just keep plodding along for another 5+ years as-is until these issues are worked out or something much better is put in place (not likely, but possible, IMO).