> Suppose your blended portfolio grows at 10%/year nominal, and you're in the 20% capital gains bracket. Then you would owe 2%/year taxes. Would you not then need an interest rate lower than 2% nominal (i.e. 0% real, assuming 2% target inflation) to come out ahead?

You're assuming that someone has a portfolio that has a cost basis that is very close to the current market value. Most ultra wealthy folks have holdings that have been held onto for a very long time, with cost basis' that might as well be $0 compared to the value of the assets. Even non-wealthy folks that saved in a traditional brokerage would have a cost basis that is very low compared to the value of the assets. During the 'accumulation' phase, there likely is not much being sold at all.

If you want to access this capital, then you're paying 20% on every dollar. You don't need a 2% interest rate to make out ahead in this situation, it can be substantially higher and still be a better deal than paying the cap gains taxes.