So, even if companies average 30% margin long term (I believe it will be much more competitive, 10-20%) it would only take ~3 million market cap across all companies to meet, if not exceed, fair value.
The quoted market cap only takes into account the scarcity of bitcoins and neglects the scarcity of transaction volume. A bitcoin is worth nothing if you can't transact it.
Unfortunately at the moment it is impossible to estimate the net worth of 50GB (max. ~1 year) of blockchain data. In any event, owning mining stocks or equipment is a way to get exposure to that speculative asset (albeit the rapid expansion of network strength depreciates equipment, making companies or intellectual property in that area a safer choice).
Thus, one may argue, a 3 million market cap for mining companies underestimates the long term worth of that sector.