I understand your point of view but do not agree because my AA didn't change one iota when I refinanced and put the proceeds to work in my investments nor would it change if I were to pay off my mortgage, so for me the relevant comparison is to the return of the pot as a whole since the refinance proceeds are effectively invested in a 60/40 portfolio.
For the same reasons when evaluating an investment decision in finance we use weighted average cost of capital rather than incremental cost of capital. Since I'm a total return investor, its all one pot... I do not try to cash flow match in any way shape or form.
I am accepting the risk that the investment results on the proceeds may be more volatile and might even underperform the 3.375% I owe on the mortgage and it might end up being a bad decision, but as an averages player who is accepting of risk I'm playing the averages with my eyes wide open.