Diversified portfolios that included international equities have diluted returns…again. There is no theoretical problem with diversification (that is how robo works, that is the global portfolio), but…tell me the scenario where international financial markets, including bonds, outperforms?
The combination of events required for either emerging markets or non-US equities to outperform isn’t probable (in the context of anything is possible). Both direction AND foreign exchange must favor this. One of them, maybe. Both? Unlikely.