"imagine the shitshow if you had to pay extra every year if you owned a house outright but the property values kept going up"
Like property taxes, then. ;-)
Realistically, I understand the issue. If I had to pay taxes on the increase in price on my house (say from a $300k valuation three years ago to a $500k valuation after the market bubble), I'd be fucked to find 15% of that overnight. Of course, if they allowed that to be offset by the primary residence exemption, it would be a zero cost. Without that, it would still be a non-issue for 95% or more of US taxpayers because most people simply don't own an illiquid asset that increases in capital value (much less an international one), and if you exclude secondary real estate that non-issue number probably increases to more then 99.9%.