Any other ideas of how to scale a crypto-currency to beyond 10 million users within 3 years or less from launch?
Build it into a game that becomes a massive hit. Unfortunately I don't know any way to engineer a game that is a massive hit. They all seem to be black swans. Most often designers of very successful games who try to repeat their success fail and end up being one hit wonders. Maybe that is not universal though, I'm not sure.
Installments of an already-successful franchise can get wide distribution but you will need to pay for that, so it comes back to advertising (a form of product placement in this case).