When I liken the presently planned economic model to a Ponzi I do it in one regard and one regard only:

if the present value of the costs collectively incurred by all the farmers to store the 1Tb of data I give them today exceeds the dollar payment I give them today

then the shortfall will need to be covered either by future customers or by the farmers themselves
Let us imagine that

farmers all become at one point economically motivated and prefer to go out of business rather than incur a loss

the annual cost to the farmer per 1Gb stored per year stabilises

banks continue paying miniscule interest on deposits like they do now
Would the survival of the network then not depend on future customers paying for the upkeep of the past data? Assuming constant dollar price per Gb stored this according to my calculation requires the number of Gb stored per day to grow proportionally to t*t where t is the age of the system (say in days or years).
Now t*t is a Ponzi, is it not? And in case the number of puts does not grow this fast the dollar price per Gb stored will need to continuously grow.
Financially this is exactly the same as if the network collectively promised to eternally pay each depositor a fixed amount of money per year per GB stored. And the price of buying such an annuity would be determined by the network based on its current outgoings (and perhaps some prediction of the future).