In terms of what to do with this info, the author offers working longer, cutting spending, or moving to a lower-cost area as the changes to make.
Overall, this study makes a good case for pursuing boring single digit returns from preferred stocks, REITs, mortgages, or bonds. Bonus: if a crash occurs, you'll know it and may be able to pivot back to equities.
I dont think you are reading the article accurately. He is saying that using current valuations in your models (such as a rolling Shiller CAPE 10) has proven to be a robust model to predict returns. He isnt saying be more conservative with the portfolio, though I know he is someone who applies typical stock and bond mix allocations for his clients to reduce volatility.
Having higher expectations does not generate better returns. Stocks are volatile...thats just fact. 100% equities is higher return, but also higher volatility of returns.
Anyway, it is known that a bond portfolio has less risk but also lower expected returns.
An analogy to his main point would be applying inflation to expense budgeting. It is like saying you should use current inflation rates in your expense budget, adjusting every year, rather than a flat inflation rate of 5%, just because that is a historic average.
Assuming expected returns is a critical part of your model to predict future actual returns. How conservative or aggressive you are in your assumptions wont change the actual outcome once you have your allocations, so I am not sure I understand your comment. Boredom or excitement has nothing to do with it. An exciting portfolio doesnt always do what a retirement plan needs. Bogle says investing should be boring and I agree. Buy, ignore, sell when you need $.
If you dont save enough or get sufficient returns to meet expenses, yes you have to make up the gap somehow.