That's not how investing works. People don't invest because they think a company is worth X, they invest because they think it will be worth X at some point in the future adjusted for the risk of not being worth X. Further, they don't necessarily buy stock from the company.
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.
Stock and Bonds are very different things. A bond is an agreement with a company to hand back X amount of money at time Y. Stock is an agreement to share in future profits.
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.
I think you are talking past each other because you are not talking about the same party when asserting whether there is or isn't a problem. It's true that the investors don't have a problem if Dropbox fails to live up to its valuation. Venture capitalists expect that most of their investments will fail. But a company that has accepted venture capital is beholden to its investors and will almost certainly have a problem if it fails to live up to their expectations. Remember that investors are part owners, and get to make decisions about how the company should be run. The more dissatisfied they become, the more drastic the changes they will demand.
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.