I agree that some use "billion dollar company" as shorthand for all the things you mention (a great lasting company, solving a real problem, and making a real impact). My point is that while it's often used as a proxy like this, it's a dangerous proxy because it's easily disconnected from the company's original mission(s). In this sense, it's an unhealthy simplification. Surely there must be a better stand-in for a startup's hard-to-measure progress towards achieving their mission than their valuation.