A company with a very large valuation needs either very large sales figures OR large growth potential. Dropbox is squarely in the second category, which is why they need to keep expanding.
In other words startups have a small chance of being worth a much larger X, and Exxon has a very large chance of being worth a slightly larger X. If you need zero risk buy government bonds not stock.
Importantly, when you buy stock from a someone other than the company then the price you pay has little to do with the company. But, again my point is risk is an inherent part of the process. If you invest in a startup and they use all of that money to buy government bonds and then fold in five years you would be pissed because they avoided risking your capital.
Those people aren't investing they're speculating. They're not there primarily to see Dropbox succeed they're shareholders because they want to get rich from other people's work.
So yes that's a problem for the company. Ultimately this model of "investment" can kill a business that is technically succeeding because the speculators don't see a high enough return. That's also a problem for society as it creates instability and prevents technologically successful companies from prospering long term.
At least that's one narrative that appears to fit well with my observations.
If you actually care about building a sustainable business, not Unicorns and moonshots and other things investors want you to become and do, don't take investment, or at least not much.
How is that money "free"?
Similar mechanics apply to having a large valuation, no?