I have read Rob's piece and agree with all the concerns about high valuations in low vol stocks (particularly when it comes to utilities, staples and health care). So long-term expected returns should be quite a bit lower than the historic track record.
But: First of all I wonder why low vol stocks have become so expensive? In my view they have become a stand-in for "bond proxies" in times of extremely low bond yields. While the situation might slowly change in the US at least here in Europe we are very far away from rising interest rates and as a result, the driver of demand for low vol bond proxy stocks should remain in place for quite some time.
Second, the question is not whether returns will be lower in the future than in the past but whether low vol stocks will outperform the overall market in the future. High valuations will depress low vol return and also partially depress overall market returns. Yet, if the research quoted above is correct and the low vol effect is independent of the value premium then it does not seem clear to me ex ante that high valuations will overwhelm the effect of low vol stocks outperforming the overall market. I think research still needs to be done to investigate what happens when different factors point in different directions.