In the case of Netflix that's obvious - new shows. Dropbox is a little more annoying, but there's still the premise the cloud component is fundamental to how their service works.
"Daily admission to the software I installed" doesn't feel like much in the way of value, even if you are continuously investing into the product behind the scenes. The equation can even become negative (e.g. forced UI revamp, user hates it, now they feel like you're charging them to make their life worse).
I get the cashflow appeal and business arguments, but I disagree it's the only way. I sold software online with a small bootstrapped team for over a decade under the plain old "buy it once" model, resisting the subscription fad even when the MBA's pushed to go that route, and made it work. My customers were always happy to buy the major version upgrades (every couple years or so) because it was really clear what new value they were getting.