Dropbox's weakening position isn't necessarily an indictment of the idea, just that they're in a tough market when you consider both Google and Apple offer (comparatively cheap) cloud storage that is well-integrated into their smartphone platforms. Also consider that the success of Google's and Apple's offerings has proved out the value of the idea many times over.
Also, Dropbox was an early mover. That worked well for them in their early days, but the consumer space is saturated by better options. Dropbox (and Box) are often better suited to enterprise use, where some of the more advanced features (that regular consumers mostly don't care about) are important.
Uber's S1 is available. They are, for sure, losing shit tons of money. And they have no reliable view into what happens to demand if you broadly raise prices. Maybe that works, maybe not. Nobody knows.
Judging by your parent's comment, HN would've been wrong to assume that the reason for Dropbox's decline is a mass movement of users towards rolling their own linux hosting stack.
I'm not saying the HN crowd didn't miss a $10B opportunity, but they also weren't exactly wrong.