What if internet culture changes in five years, and the journalist loses her job because of this? Sure, you can delete a tweet, but people are backing up Twitter at least daily, and they can even run a weekly "diff" comparing the latest version to the database file from 7 days ago, sort the results by popularity of user, and boom. You have a product. Yes, there are all sorts of tricks you could use to expedite the computation process, I don't care about implementation (publicly).
I feel like anonymous and encrypted online interaction is becoming increasingly incentivized. Encryption: meh, a little inconvenient, but there are some serious drawbacks to incentivizing anonymity, including losing the pseudonymous space we enjoy here, maybe.
Pretty good comments below the article.