The fuzzy math is what asset class would you put it into the capitalize on that? Almost zero are returning inflation, so it makes no sense if you actually plan investments for a living. But I get that everyone repeats this logic, with no clue how it works.

The problem is paying back the principle, I would bet a lot of those carrying huge HELOC balances are expecting to pay it off when they sell or refinance. With the market on a downward trajectory that could snowball into a problem when their equity has diminished so quickly and will likely stay low as the market finds a bottom.