It can't go parabolic forever. Once it starts hitting certain valuation, other coins such as litecoin will become more popular as they'll be more affordable to the average user.

This is unit bias.

The idea that we have to buy a full coin is a grand fallacy.

Using the same logic I could tell you that one dollar buys 27000 Satoshi.
What an amazing deal amirite?

In addition, this isn't parabolic growth, this is exponential growth, which is much much stronger than parabolic.

parabolic: x^2
exponential: 2^x

Moore's Law has been going strong for 50 years now, so the idea that Bitcoin can't keep doubling every year because it just can't isn't really founded on anything except the gut feeling that exponential growth doesn't really make sense to us.

If Bitcoin had the same market cap of gold (7.8 trillion) each coin would be worth $446,000. In the opinion of many, Bitcoin's value far exceeds gold. This is smart money we are talking about. The sky is the limit. Power over money and the corruption it breeds is about to come to an end, and the value of that is priceless.