Currently worth $70 Bn, Is this a lot?

Personally it seems quite a lot for a fairly untested currency.

US gold reserves currently worth something like $300Bn, the largest in the world (though it's no clear if anyones checked its there). All this is less than a company like Google but still $70Bn is a lot of money.

For the bitcoin price to rise  a lot of money would have to flow in. Right now it seems like every grandma and her dog is being persuaded onto the bitcoin bandwagon. Is this not a sign of an impending crash, in which those at the bottom of the pyramid get burned? Is the bitcoin rise sustainable? Where will the money be flowing in from? Will shops have to start selling bitcoin investment scratch cards over the counter to keep the price rising?

I'm keen to head from people who can make arguments based on fundamental economic principles, either way.