There is no fundamental worth of bitcoin - just like there is no fundamental worth of dollar. You exchange your work or goods for dollars because you speculate that you'll buy something else with it - there is no use of dollars as such - they are only used for exchange for something else.
The fact that something is used as a means of exchange is a part of its fundamental value. If, e.g. oil were no longer traded in USD, its value would go done. Because fewer people need the dollars, even if just temporarily. The "temporarily thing" relates to velocity. In the real world people usually get paid monthly, and spend a large amount before the next month. So, e.g. if 2 million people had their wages paid in BTC and had an average wage aquivalent to $5k the market value, for purely fundamental reasons, would have to be at least $10B (otherwise not everyone could get their wage at the beginning of the month)