Keith, a lot of the problems you cite result from the way OTEC is approached. The conventional way is to use massive pipes to bring cold water near the surface to condense the working fluid. Their size means the surface infrastructure also has to be large and surface heat is diluted making for the potential play out you suggest. There are also environmental problems associated with moving this much water and bringing water containing CO2 dissolved under pressure up to the surface.
The heat pipe design uses piping one order of magnitude smaller, 1 meter versus 10 meters for a 100 MW plant, thus the costs are at least 30 percent lower and the parasitic losses are halved. The Vega study, which is for the conventional design says electricity can be produces at a cost of less than 0.18 $/kWh, which is about the global mean.
It is understood that the thermodynamics dictate that any plant less than 100MW will not be economically viable.
For the resource to play out using a heat pipe would mean surface heat has been reduced, which is precisely what the world needs. As Rahmstorf suggests the warming of the deep would be neglegible so the resource won't dry up on account of moving heat there.
Using CO2 as a working fluid and the heat pipe design these plants can be scaled to gigawatt capacities.
The group I am working with believes the reason OTEC has never been done on a sizable scale is because it has consistantly been approached in the same, and the incorrect, way.