[wpietri]

Not at all. It's a recognition that the people who made long-term arrangements under the old law shouldn't be punished for that. The safer people feel making commitments, the more we can pursue things that take time to pay off, which has broad societal benefits. And in practice, laws are much harder to change if you piss off a lot of people who didn't do anything wrong but now suddenly stand to lose big.

As an example, take the mortgage interest tax credit. Personally, I strongly believe it should go away; if the government is going to subsidize housing, it shouldn't spend most of that money on the already well off. But I think it would be wrong to screw all the homeowners who bought a house expecting the credit. We'd see a wave of disruption (short sales, foreclosures, people suddenly barely scraping by) that benefits nobody. So I'm fine with grandfathering existing mortgages and gradually phasing out the credit.

[jonahhorowitz]

It would be totally reasonable to phase out the mortgage interest deduction over ~10 years, giving people time to plan and decide how they wanted to handle it. The cap is currently $1M, let's reduce it by $100k/yr. (or $50k/yr over 20 years)