Apart from the write cost, there's some engineering issues you may be interested in. Besides error checking/correction, there's persistent storage. DNA is fairly stable when you compare it to things like proteins and RNA, but it still gets unhappy at room temp if left out long enough (even if embedded on cellulose or silicon). So far as I know -80 C is still the standard for super long term storage, which is a huge energy and reliability cost. Propagating it as a clone is asking for all sorts of issues with maintenance and mutations.

Also, it should be understood that this would be a high capacaity, but also high latency, storage method. More like a tape drive than a usb stick.