Note that your 8 year number has 2 assumptions buried in it: (1) that batteries are discharged/charged every day, and (2) that batteries need to be replaced onc

Note that your 8 year number has 2 assumptions buried in it: (1) that batteries are discharged/charged every day, and (2) that batteries need to be replaced once they reach ~ 70% of their original capacity. Neither of these assumptions is apply to grid-scale storage. For (2), you can install 30% new capacity and keep the old batteries in service. And for (1), the size of the battery storage is dictated by wanting to last 3 cloudy days... which rarely happens. So you might only cycle 1/3 of the batteries on an average day.