If the biggest fear is financial ruin due to running out of money in retirement, why use a 30%/70% stock/bond portfolio to prove your point when it fails 10 percentage points more often than a 50/50 portfolio? I'm not necessarily advocating any of these allocations, but what does their MC simulation look like at 70% stocks? 90%? 100%?
We can't recreate the Betterment MC simulation, but we can do the Vanguard one:
2.7% SWR 40 year retirement
Stocks/Bonds Probability that the portfolio survives 40 years
30/70 99%
50/50 99%
80/20 97%
100/0 95%
4.0% SWR 40 year retirement
Stocks/Bonds Probability that the portfolio survives 40 years
30/70 81%
50/50 86%
80/20 85%
100/0 83%
Vanguard's tool isn't much different to Betterment's advice.
If a person takes the 4% route they have to be sure the budget can allow for spending cut backs, just in case.
For me the 'stache is more-or-less complete. I'm trying to decide how ridiculously extravagant and philanthropic we can be for the next 40-50 years.