Bitcoin doesn't need investors. A good currency has very little investment. Bitcoin will have succeeded when nobody wants to invest in it anymore.
If there are no investors, then the price will roll down, always down. People who receive cryptocurrencies for any actions will sell it, and if there are no investors, then there is nobody to buy, which means a drop in prices. Investors are needed.
How many people invest in dollars, or euros, or yen? Not many, yet that doesn't seem to affect their values. Anyway, only the investors care about the price and investors aren't needed.
Invesing in fiat such as dollars, euros or yen is different from investing in bitcoin.
The value of fiat remains because the government maintain its value. While in bitcoin, the price determines based on its market demand (investors) and its supply.