Did I say mandatory? I meant optional! You’re “free” to die in a cardboard box under a freeway as a market capitalist scarecrow warning to the other ants so they keep showing up to make us more!
Capital gains are applied against a cost basis, in the case of your homeowner, their purchase price. Unless the house appreciates in value there is 0 capital gain, even if you made the mortgage a realization event and for some reason implemented this with no residence exemption or tax brackets. It’s mad how this point has to be repeatedly explained through this thread.