But to enter the market, you just need to buy those Bitmain ASICs and find a cheap power source. That's what makes it a commodity market: barriers to entry are low, so if a big player gets too dominant, a bunch of new entrants with money but not necessarily expertise can take them down. You can't really do that to say Goldman Sachs or Wells Fargo, where a large portion of the competitive advantage they've built up is the trust (har har) and relationships they've developed over the past 150 years.
There's perhaps worries that Bitmain could get too big and poison the chain, but they're hamstrung in that by Bitcoin being an open protocol. If they did that the network would fork, and despite the higher hash rate in the Bitmain fork, holders would dump the Bitmain coins and buy the original fork, since they wouldn't trust the poisoned chain.
