Hi Marley - it's rare that I disagree with you, but there *might* be a good reason why Woodward waited. I'll present arguments for him first...and then at the end I'll show why he still could have released the tapes (if he - and not his publisher - had custody of them).
In 1971 - the year before Woodward and Bernstein began their investigation that led to Watergate - Nixon had Daniel Ellsberg arrested and tried under the Espionage Act, and Nixon's actions led to the secrecy that Woodward and Bernstein strove to maintain until their investigation was complete.
Thing is, it's already been noted that while Nixon at least tried to abide by some governmental norms, Trump isn't bothered by those norms at all, and unlike Nixon's AG Richardson (who refused to fire the Special Counsel Archibald Cox as Nixon had ordered), AG Barr has essentially acted as his personal attorney.
With that in mind, Woodward would know that he faced a wholly different level of threat from Trump, one that might lead not just to arrest under made-up charges just like Daniel Ellsberg was, but possibly worse.
I don't know if that is the case...but it *might* be. We don't know and cannot know for sure whether Woodward waited for mercenary reasons, or if his intention was to make sure enough of the information was out at the right time to ensure his personal safety.
What's more, if he'd released the tapes as soon as he'd finished recording the 18th and final interview, he may well have not only violated a contract with the publisher bankrolling his interviews, but doing so may also have violated journalistic ethics and thus ended his career.
Yes, Woodward may well have sacrificed his reputation and career by releasing the tapes early, and also given Trump more of a chance to respond and cover for himself.
But as you said, he would likely have saved tens of thousands of lives. Furthermore, he wouldn't be the first journalist to go against his publisher's wishes and risk his career. Ronan Farrow did by ignoring his publisher's orders to stop. He took his information to another magazine and so gave rise to the "Me Too" movement...and so cemented his reputation as someone who fights for right regardless of what it does to his career.
Woodward could have done the same, and he certainly didn't need the money.
And as I've said so often before, I wish I could write with half the passion you do.