Dropbox has been operating at a loss since its creation and is currently not making a profit, so the jury is still out on whether it is actually successful (or just a VC-subsidized service that got advertised a lot).
I would much prefer to build a Ford than a Yahoo/Tumblr/Compaq, even if it meant my personal fortune was 10x or 100x lower.
It hasn't always been that way at all, but it certainly is now. I think this is one of the huge red flags about the modern incarnation of the tech industry.
If dropbox only had a high valuation on paper, but employees/founders/investors couldn't actually exchange that into real $ in their bank account then sure, you can say the jury is still out.
But that's not the case. They built a product, have tons of paying users, and have IPO'd with a market cap over $10B. It's not money that only exists on paper. Founders and employees can sell their shares for real money and buy real things.
Their shareholders aren't just a handful of VC's anymore - they are public. The jury has decided, it's successful
Hypothetically, what if it never begins earning more money than it spends each year? At some point wouldn't it be unable to borrow any more (from banks or investors) and be forced to shut down? And if so, we still don't know if it can be profitable, right?