Opportunity Cost

Opportunity cost is invisible on any statement, which is why it gets ignored. A paid-off house never sends a bill for the stock returns its trapped equity did not earn, and a low-rate mortgage never itemizes the flexibility it costs to keep. The only way to see it is to model the alternative explicitly.

That is how comparison calculators handle it: the sell-and-reinvest path in a rent-vs-sell projection is nothing but the opportunity cost of keeping the house, made concrete at a stated reinvestment return. The number is only as honest as that assumed return, so test it with conservative figures before treating the gap as real.