As someone who's tried funding a company that way, I can tell you it doesn't work.
Companies like Google, Facebook, Dropbox and Airbnb all looked like crazy ideas to begin with. Ordinary people with limited funds shouldn't be investing in companies like these, as they fail far more often than not.
It's only due to the existence of high-net-worth individuals, who can afford to lose money on most of their investments and who have a "pay it forward" mentality, that these companies can get their start.
If the returns for people Page and Gates and Zuckerburg were capped at $10M, there's a good chance they wouldn't bother.
For people like this, the goal is not personal enrichment but rather the ability to better the world, not just via their products, but also through philanthropy.
Take away the blue sky returns, and you take the motive that is most beneficial to the rest of the world.