Just imagine what market cap should we get if a single bitcoin would cost 1$mln. That numbers would be able to compete with the other fiat currencies, which tbh doesn't seem realistic. And what should happen to make people invest in cryptocurrencies at least the same amounts as half of its "potential MC"? Probably nothing because it wouldn't happen.
The larger MC is, the smaller growth it will provide. It is easier to buy a pen for 1$ and resell it for 1.5$ with a 50% growth but you won't be able to resell it each time for 50% more.