How illiquid is it given the widespread availability of HELOCs?
HELOC is a loan and doesn't have anything to do with how liquid the asset is.
Banks have lending limits. You're assuming the $2mm house isn't leveraged to begin with.
I’m sure you can find some obscure edge case somewhere but by and large people that own things that are worth a lot of money are rich and don’t especially need our sympathy. Let’s save that for all the actual poor people out there that don’t own houses, much less multi-million dollar houses.