The CBA platinum amex/MC combo is the best one CBA can currently offer. There isn't yet a higher level one that you unlock with a high enough income.
It's not a particularly good platinum card because while the amex converts at $1 to 1.5 QFF points you're capped at a ridiculously low $40,000 spend after which the conversion rate plummets. So I use mine for the first $40K spend and then use other cards. If I wasn't getting it for free I would cancel it.
However, I did hear a whisper recently that CBA is aware that their platinum card is unattractive to high spenders and are about to release a higher level "black" card which may or may not be called a Diamond card.