I can't say for sure whatever the reasons may be for those big players to power down some of their Steem. It could range from a tactical move, a business move, or they are just trying to liquidate their SP so they can transfer it to their alts.
It is understandable that if you only look at a few accounts (especially the big players), it might seem they are losing interest and hope for the future of Steem. However if you look at the Overall Power Up and Power Down difference for ALL Steem Accounts in the platform, it seems lately that a lot of people are powering up not powering down.
The image below was taken from penguinpablo's latest weekly report for Week 35 (Aug 28 -Sep 4) of year 2018 (DATE).
So No! It seems like a lot of people still have high hopes for STEEM especially after they announce Hard Fork 20 and the possible release of SMT next year.