Rhetorical question, BTW. There is no market mechanism that determines what is sufficient security. This should become clear if one looks at what the network is choosing to pay the miners who secure transactions.

The block reward is now 12.6% of what it originally was, and tx fees have certainly not gone up correspondingly. So is the network now undersecured? Or was it previously overpaying for security? Again, rhetorical questions - the users of the network aren't paying for security, they're paying to have their transactions processed and have no choice but to subsidize the miners with the block reward as well. As far as how much security is needed goes, the Bitcoin answer is simply "more is more."

But in the future, as the block reward dwindles, won't market forces start to price in security? I'm sceptical, personally, because the only gauge of whether or not the network is paying its miners enough is whether or not the network works. So if, in the process of finding the proper price of security, the market goes too low, attacks will be possible and the price of BTC will likely suffer as a consequence. This would make it less attractive to mine BTC and less attractive to use BTC. You can see how this could lead to a downward spiral.