You won't see battery costs stop dropping until the market is saturated- at which point you can no longer gain cost-savings from economy of scale since any further scale-up would lead to overproduction and the inability to make a profit.
We're still way above where batteries COULD be priced - and you can assume your first replacement to be significantly cheaper than the initial installation (which has been the experience of most buyers already).
Furthermore - what we ARE improving in terms of the technology of batteries is lifetime. So the second set will likely last 12 years. Right now you say "10 years" but the first home solar battery-packs only lasted 7. We've already increased it by 3 years in under a decade.

And of course - if we ever crack hydrogen battery tech, we'd be able to put 3 times as much power in the same volume using the cheapest, most abundant material in the entire universe. 
People who actually design powergrids say you are wrong. Maybe they know more than you?