I think the part that needs to be factored into your analysis is the volatility/risk aspect. A mortgage may be relatively low ROI but it’s also relatively low risk compared to the market. E.g., maybe somebody has a certain percentage invested into low risk bonds. Maybe it makes sense to pare back some of that and put it towards their mortgage during a period when bonds are being crushed. Neither bonds or the mortgage return will compete with a general index fund in terms of return over a long period of time, but the index fund is in a different (higher) risk category