Well I like the idea of there being less rigidity than there is now. I do think that such an exchange would buy SP at a discount, so that would still provide some incentive to power down the 13 week way, thus continuing stability in the platform.
My only concern would be with where the market might set the price. Would it be so low that it is basically extortion of people when they are at their most vulnerable and must take any offer? Or would it be so high that there is inadequate incentive to ever power down over 13 weeks and this simply becomes the new way of cashing out, with all the problems you noted of no power down period?
Is it really safe to presume the free market always prices things in a way to achieve optimal outcomes for the platform?