I think you need to consider the idea that this is the way he's always acted.

His handling of the Grindelwald business can be summed up as several years of inaction followed by the most spectacular duel in recent history. Presumably he didn't explain the blood sacrifices + Deathstick = invincibility thing to everyone who asked, so he must have skated by on inscrutability.

He states with a certain bitter pride that he taught Voldemort he doesn't give in to blackmail or threats to hostages- which he (hopefully, considering the alternative) accomplished through more inaction.

When you think about it, a wizard with tremendous magical and political power who doesn't seem to actually want to do anything with that power is pretty much the best case scenario for a lot of people. Imagine that Dumbledore suddenly decided to act on Fawkes' advice: how much of Wizarding Britain is left standing?