You can "open" multiple stores in a transaction so there's no need to have dozens for a dozen stores. There's some onus on you to work out the batching as IndexedDB is quite low level for what we usually get in the browser.

> But FileSystem API seems to finally getting broad support.

I'm not seeing much beyond Chrome & it's associates for it?

That said that could just be because "filesystem" as a word is mentioned in a dozen different web APIs from allowing file input reading locally, to the deprecated API chrome had for local file access, to the new thing that came out of "project Fugu" for PWAs to have native file access and I'm missing which one is which in terms of support & docs.