I mean if the megapack in Australia paid itself off so fast it kinda suggests your assumptions are wrong.
Could be, though its a bit of a stretch to consider a mega utility scale installation in a near worst-case grid environment (thanks in no small part to the legacy power suppliers) as the benchmark for such an endeavor. Your point is
really that a mega utility scale battery can successfully augment The Grid, and I'd surmise that nobody disagrees.
But, to wit, for the foreseeable future you're not going to [legally] sell power to your neighbor.
As a few data points: 1) Hornsdale is operated by Neoen and power is distributed by ElectraNet, 2) The Monterey megapack is commissioned by PG&E, who will presumably operate (or subcontract operations) and distribute the resources.
Edit: And the early projects are likely to have a much better ROI than the later ones. Not much profit when the duck curve is almost gone. compared to when its causing system stress, like now in california
On the flip side, the later projects are much more likely to have better ROI than the early ones because of 1) ever decreasing capex as storage technology improves
and storage production increases
and as storage demand creates competition amongst suppliers 2) ever increasing acceptance of the concept enabled by evolving sociopolitical and bureaucratic mindsets (see: solar), enabling more favorable distributed power solutions, and 3) ever increasing demand due to ever increasing renewable installations that, without corollary storage, ultimately asymptote at some demand that necessarily relies on energy generation from more traditional sources.