It depends. If you're just a big-audience crossposter on multiple platforms who doesn't really engage on steem.....sure no biggie.
But if you've made real connections here it's really just a common courtesy to those you've met and supported to maybe explain and say goodbye.
It's got nothing to do with stake or powering down. If you've earned your stake legitimately and independently then its yours to do as you please.
I'm with you 100% on the final point. I left stake (and how it was earned) out of the post on purpose but I do think people take a differing view towards leavers that have legitimately earned their stake only to leave when the chips are down, or even when the market is booming.
Common courtesy, and a chance to close a chapter for all parties.