It sounds to me like this Cornucopian belief is just the same 'let the market handle it' position with better branding. So the counter-argument also remains th

It sounds to me like this Cornucopian belief is just the same 'let the market handle it' position with better branding. So the counter-argument also remains the same: 'the market' moves in fits and starts, and there is very real and severe socio-economic cost to disruptions between the start of price shocks and the emergence of alternatives. It's not unlike advocating a central bank to (theoretically) smooth out natural business cycles, not because the market would implode without it, but because you believe you can fill in a bit of the extreme troughs at the cost of taking a commensurate amount off the extreme peaks.