It is a replacement, but I'm not really sure whether it's a good one. SQL was a nice choice because it made writing hybrid applications (with online and offline DB storage) very easy, and it was using an API that was already well known and understood. At this point it's unclear if IndexedDB is actually going anywhere, it's a prime example of a lifeless spec being prescribed from "high above". It might get adopted, maybe browser makers will force web devs to do it in the end, but SQL already has been proven more than workable in the field whereas IndexedDB has not.