They're a person, not a tenant.
They do not exist in isolation within the household. Even less so than a tenant.
They have a right to their short-term life
preferences, but they don't have a right to
achieve them at someone else's expense, in terms of time, energy, mental stress, or resources.
So broadly speaking, they either need to fully support themselves, or they must
negotiate their place.
"Full support" isn't a
requirement of course. But it's the way to achieve the most freedom and independence.
Similarly parents have both a legal and moral obligation to participate in the negotiation, particularly with a troubled child. But that's not the same as a "blank check". The negotiation is to achieve a reasonable balance of
everyone's interests, within the available resources and capabilities.
If either party "stonewalls", they are misbehaving. It implies imposing unnecessary and unfair costs on the others without any compensation. All young adults do this to some extent of course - they're naturally quite selfish, and can't be criticized too much for it. But there are limits.
This isn't for babies and very young children of course. But somewhere along the way to adulthood, the child has to become a more of a participant, and less of a dependent.
BTW - We can stop at any point.
I know I'm indirectly criticizing your past, but it's a little like the story of the Frog and the Scorpion - I can't change my nature. I'll argue for my honest opinion (which
can change, but it's not likely in this case), or I'll stop.