Apologies in advance if the article in the link below has been posted already here, but it was a new article to me on the same subject and addresses the question of how accurate have Bogle's forecasts been in the past:
Bogle: Stocks 6%, Bonds 3%
"Formally, the relationship between Bogle's 10-year stock forecasts and subsequent market returns is 0.81. The even-simpler bond model has an even-higher correlation of 0.95, illustrating once again that with investment projections, less can be more. (Greater complexity does not necessarily mean greater sophistication; as Mark Twain wrote, sometimes people compose long letters because they lack the time--and perhaps the skill--to draft a short one.)
Thus, the evidence suggests that while Bogle's forecasts aren't reliable guides for a single, specified time period, they will likely be close to the mark over a sufficiently long term."