The mortgage interest deduction serves to put prospective owner-occupants on an equal footing with prospective landlords (for which business loan interest is deductible against profits).

The capital gains exclusion I have less strong feelings about, but it amounts in large part to excluding being taxed on the effect of inflation.

The MDI defines "owner occupant" over "landlord" in such a way as to effectively subsidize the single family detached suburban home lifestyle preference. It encourages the middle class to tie up a big chunk of their net worth in sprawling developments with questionable claims of being an appreciating asset.

I’m trying to gain understanding of your first sentence, but have been unable.

Landlords can deduct mortgage interest just like any profit-seeking business can deduct interest on loans taken on in furtherance of their business. That’s true for single-family homes or duplexes in a suburb same as a rowhouse, 4-plex or 50-unit building in a city.

The owner-occupant deduction applies to their unit, whether that’s all of an SFR, half a duplex, one level of a triple-decker, or 1 unit in a 4-plex.

The mortgage interest tax deduction is effectively eliminated for 90% of Americans because the standard deductions are high enough that 90% of people do not itemize to be able to claim the mortgage interest tax deduction.

Great let's simplify the tax code and clear it out completely. It's never been good policy.