Investors who followed their forecasts have missed a historic bull market. Of course, it is the economy's fault, not their models.

I think there's a misunderstanding here.
Don't think of them as forecasts--the market can do anything.

Think of them instead as what a reasonable person should expect. If you remember that, they're spectacular and directly useful work.

They explicitly assume that valuation levels will be at some kind of "normal" at the end date, similar to the average in the past. Maybe they use too much or too little history, but that's a quibble. They key point is that one should never expect more than that. If valuations expand to unusually high levels during the term addressed, then returns will of course be much higher than the figures they suggest. Sure, it might happen, so it would turn out to have been a poor "prediction". But what sane person would plan on that assumption?

When you hear hoofbeats, expect horses not zebras. More importantly, since the future is unknown, it's best to arrange your financial affairs assuming horses as the best available guess.

At the moment, anyone planning on decent real returns for the S&P 500 in the next several years (say, over 4%/year) reminds me of someone embarking upon a fourth marriage: a triumph of hope over experience. Might the returns be good? sure. Does anyone rational expect that? Nope.

Jim