It's a housing version of the national debt philosophy, where debt doesn't matter as long as you're outpacing it with growth. If last decade you had $25k HELOC on a 400k house and today you have $50k HELOC on a 800k house, your finances have clearly improved

Except that you still can't sell the house or you won't have a place to live, so they've only improved on paper before you account for the opportunity cost of the higher imputed rent, i.e. the higher cost of living. Meanwhile you could have gotten the $50k HELOC against the $400k house, which was the only part doing anything that would actually affect your life.