Very interesting observation. Seems the debate as to why these larger accounts are in 'Power Down' mode will heat up the more it becomes noticed.
It could be due to the censorship issue (even though I've been hammered fairly hard on that topic nearly as much as I have on the subjective subject of 'spam' ;) ), even though the White Papers state censorship does not exist in steemit for the users. It could be because they are bored or fed up with the platform, or maybe even planing to move on to an alternative. Or it could even be they earned enough to get that new car or home they dreamed of when they first started here.
Yet after reading the White Papers and having a better understanding of the capability and multiple purposes of this platform, I am confident it's not going to die because a few Whales cashed out.
P.S. Side Note:
I owe you an apology for all that b.s. I pulled in craig grant's posts. I did what I did based on what I was observing without the knowledge of the White Papers, and now see he (and many others) are only doing what the White Papers have given a blessing to - what and how they operate here. At least you put in an effort to set me straight, despite flagging me, and pushed me to read those White Papers to better educate myself & to work to make this account successful despite my mistakes- Thank You @avva.