The "tax expenditure" of mortgage interest deduction is a political 3rd rail -- removing this deduction as it applies to existing housing would severely harm the middle class. Yet, the deduction is grossly unfair to Americans who rent -- why should renters subsidize purchasers via the tax code? As a matter of fairness, we should preserve the deduction for those who've already budgeted it in, while removing its pernicious effects for the future.
As a secondary matter, the short-term boost to property values will likely allow many middle-class families to refinance their homes, allowing them to take advantage of low interest rates that their houses' valuations don't allow them to access.