I was expecting some drop in sales after Destiny 1, though maybe not to this extent.
Destiny 1 was both a swan song for last Gen and part of the arrival of this Gen (arrival like Gears 1 was for the 360 gen. This system is here and is firing on all cylinders). It was hyped as a once in a (console) generation event!
I also feel that also was vague enough in what it was that people read what they wanted into it. Sure it was a shooter, but also loot, and rpg, and epic story, and multiplayer, and and and... there was just enough and just not enough shown to lead one to believe it was whatever you wanted.
In short it was a perfect stoem of hype! This led to the astronomical sales it received, more than it deserved, though it still deserved a lot. It punched above its weight. Then when people saw what it truly was it turned out to be not what everyone wanted, just what a large number of people wanted.
Destiny 2s sales turn out to be Destiny punching at its weight. It's no longer the generational event, "just" a normal large AAA game launch. Maybe even dampened a bit due to the backlash of its predecessor in some group.