Currently worth $70 Bn, Is this a lot?
In case of bitcoin with its limited suply only demand limits the market cap. The current market cap is big but is is still not as high as it can be. just imagine how many new users will come when BTC would be widely used all over the world. That will significantly incease the demand and increase the bitcoin price and total market cap.
Yes, long term. But bitcoin has risen sharply. If it's real value has not risen as much then it's likely a bubble caused by people investing because they want to make money (fiat) and intend to pull it out. Then we could see the price fall sharply, for a time until bitcoin starts demonstrating it's value more.
I'm interest reasoning about the value of bitcoin, and the available pool of money, from the more mainstream 'gambling' investors who are not invested in the economic philosophy.