> “you’re an adult once you realise you’re it; you can fail; and it’s nobody else’s job to make sure you don’t”

There's an embedded ambiguity in your sentence in that there are two kinds of failure. One is a failure of endeavor where one don't get what one necessarily wants but who has the basics. Two is a catastrophic type failure with some combination of deprivation, insecurity, and health problems as a result.

Most people would agree with your statement when it comes to one. When it comes to two I don't think such a consensus exists.

My interpretation of the disagreement you are replying to is that possibly differing definitions of failure are being used.