At some point the escalating price does cause the curves to intersect; Russia didn't need seawater at the prices it would cost to build canals.
Desal plants typically don't have dirty water shipped to them, they are located where dirty water is. The limited resource consumed by their construction isn't seawater, it's location.
Which matters why? If you can put the desalinization plant by the ocean and pipe the clean water 85 kilometers to the city (as Australia is doing now, and similar to a planned canal in Israel), the oceanwater still is being consumed.
And consumption of the infinite ocean water is still higher than what it would have been if the supply were smaller. "Supply creates its own demand." Not a hard concept.
Finally, I'd like to ask: how on earth does any of this undermine my original point that cryonics patients increase an economy's capital by not consuming and hence increase long-term growth and net wealth? These are either accounting identities or borne out by many centuries of economic growth, and is a trivial claim.
A suspended person is economically as productive as a nonexistent person.
Nonexistent people have had zero impact on economy historically. Why would suspended people have an effect in the future?
And if supply created demand, there should be a huge demand for seawater, rather than a small demand. As it is, the demand for seawater is pretty much inelastic with supply.
Imagine you accept the main idea of cryonics, that if we freeze brains future technology is likely to be able to extract the encoded information and revive the person digitally. [1] While this currently costs about $120K/person, if we did it routinely to everyone it could probably get down below $1000/person. Which is interesting: the current cost of averting a death is around $1700, but someone who doesn't die of malaria is still going to die of old age, so you can't really say their death was "averted". While someone who is revived after being frozen wouldn't live eternally, they might get to experience thousands of years of subjective life. In terms of life-years, getting cryonics to be really cheap and paying for people to get it sounds like it beats GiveWell's top charities.
Aside from not agreeing that cryonics is likely to work, however, I don't think the value is actually all that high. A future world which would revive large numbers of people we freeze today would be massively different from the current world economically, but would still have constraints. There would be some number of digital people that could run simultaneously on whatever people-emulating hardware they have. Preserving additional 21st century minds would give future people the option to run revived people instead of new people, but I don't think it affects the overall number emulated.
[1] I don't think this is likely.
I also posted this on my blog.