All assumptions in investing are arbitrary, whether you use MPT or not. Goals based investing can help address behavioral tendencies, but you still have to build the portfolios for each bucket. For example, you mention your long-term superannuation would be 100% equities. Would it be globally diversified or all domestic? Large cap? Small cap? Determining that would require arbitrary assumptions. One could certainly decide to weight by market cap, however if a more efficient solution was available by optimizing based on risk, return, and correlation, wouldn't one want to do that?