[astrange]

The best approach there isn't to lower taxes (although a deduction for a primary household is fine of course) but to allow them to defer payment until it's sold/they die. Since in this situation their house is also worth several million dollars.

[spiralx]

Which then significantly exacerbates the potential for a negative equity trap, because moving now requires paying all of those deferred taxes. So the longer you've lived somewhere the more you need to down-value when you move...