My first reaction to this is that the climate has changed since then. The use of the Espionage Act and nonjudicial targeting of reporters and leakers has been on the rise for a few decades; it's not just Assange but Thomas Drake, John Kiriakou, Joseph Wilson, and Barrett Brown. If James Bamford published the equivalent of
Inside the Puzzle Palace today, I'd fully expect him to face prosecution. The softer reaction to the Panama Papers had less to do with protections for responsible publication than with the leaker never being caught, and the near-total absence of notable Americans.
But honestly, saying earlier leakers did fine also seems pretty selective. Deep Throat did fine because he kept his identity secret for 30 years. Woodward did well because his publications forced Nixon to resign, producing an administration that wasn't being directly harmed by the leaks.
Before Nixon's resignation, those associated with leaks suffered quite a bit more. Daniel Ellsberg had his medical records stolen from his psychiatrist's office. When Nixon thought the Brookings Institute had Vietnam files, he ordered their office robbed and later planned a firebombing to enable an FBI raid; this was apparently preempted only because the Pentagon Papers were published. Other reporters and editors were regularly targeted with tax audits for criticizing Nixon. Martha Mitchell was kidnapped and sedated to prevent her from discussing her husband's role in Watergate. Many of the people targeted most heavily, for instance for FBI harassment and break-ins, were activists instead of reporters, but Assange's place on that continuum is hardly clear.
It does seem excessive to claim that this was an inevitable outcome. But I don't think the recent or Nixon-era experiences of leakers justify any belief whatsoever that responsible leakers face less retribution.