It would definitely leave a huge gap in transpacific flights for oneworld… with only JL’s limited frequencies (with connections made even more difficult with the HND/NRT split) + AA’s anemic Asian coverage. Whereas Star would have UA, AC, NH, CA, CX, BR and SQ.
The whole HND/NRT split never made much sense to me. I wish Tokyo would just have one gigantic airport with cheap and fast access to central Tokyo. I don't think JL can replace CX as well.