Yes, this is one of the biggest issues. Each percentage of growth requires even more and more money to flow into the system, compound this with about 3600 new BTC coming into existence each day and the money needed for growth starts to loom large. As an example, just for prices to stay the same this month, we would need over $50M in new value to enter the system (assuming that about 1/3rd of coins are just simply off the market). Mind you though, the new value doesn't have to come in the form of fiat; it could come in other forms. But to acquire $50M each month isn't something so easy. And remember, this is just to maintain the price. Growth would require proportionally greater and greater amounts of value.
$50mil a month is nothing for bill gates and that is ONE person
there is plenty of money out there, believe me
Of course there is plenty money out there. (The US M2 Money Supply is at over $10 Trillion.) The trick is convincing people to put money into BTC over, let's say, finding treatments for cancer (since you used Bill Gates as an example). Gates definitely has no trouble donating billions of USD to charity, but I'm fairly confident that he would be significantly more hesitant putting money into BTC. Just because there is money out there doesn't mean it's going to go into the bucket that you think it could go in to.