The argument by analogy, as I understood it, was the following.

  Internet idealism : Internet negatives¹ :: longevity idealism : longevity negatives²
  ¹flaming & porn ²scarcity & overpopulation

In the context of a discussion about whether or not to undertake a societal change (widespread longevity), bringing up an example of a prior societal change (The Internet) where the result didn't live up to the ideal is more likely than not an argument against making the change in favor of maintaining the status quo (killing statistical 72 year-olds).