I was surprised that even at level 5 it still says "stuff isn't getting dropped." Might I suggest starting to drop stuff well before that level? Yes, on purpose. It can even help you find out what's really important and who actually is relying on you to do all that stuff.
Overall, this sounds like it was posted by a person with an over-active sense of responsibility, which I can relate to all too well...
As someone who's work days are always level 5, and occasionally level 6, I think that transition (5 to 6) is the point where you absolutely need to ask for help. 6 inevitably leads to 7 if uncorrected.
At many workplaces, it's totally OK to drop things deliberately. I've been told by managers that [Company] deliberately loads you with more than you can do and expects you to make the right judgment calls. It's even sometimes OK to drop them by accident as long as you know they are being dropped and communicate the droppage after the fact. When you start dropping things and don't even know you're dropping them, that's usually where it starts becoming a problem for the organization and where it starts coming up in performance reviews.
Yes, it's well observed, but the fact that only the top level is considered 'unsustainable' is an issue - level 4 is already sub-optimal as there is no margin. And there are 3 levels above that.
Overall, this sounds like it was posted by a person with an over-active sense of responsibility, which I can relate to all too well...