I think children in general are overprotected and infantilised. The myth of perpetual happiness as being a good thing undermines resilience. As a consequence older teens and young adults have lost the skills to be independent, to an extent.
That said, I think its far, far more detrimental to leave them to flounder and cope unassisted at eighteen. The age of majority reduced from 21 in 1970. I’m not sure that was entirely helpful to the way society treats young adults. Children who have ongoing support as they transition are far more likely to embed successfully into the adult world.
Anecdotes about standing on your own two feet with two children and no money aren’t exactly selling the idea that full abandonment of parental involvement at eighteen is in anyone’s best interests.
It’s also not either or, is it? You can be holding down a very responsible job but still need emotional support from your mother when you’ve sat all night with a dying child and their family. You might be doing a brilliant job doing up a scruffy flat having scraped together a deposit but still not be a terribly good cook and want a roast on a Sunday.
You might suffer horribly with homesickness when you are at a foreign university or deployed for nine months and calling home for a chat might keep you grounded. Even nicer is your parents come to see you, no?
Why the rush into adulthood? Children born to very young parents have much worse outcomes. Children forced into part time work alongside studying have worse outcomes. Children starting sexual activity or drinking alcohol at a young age have worse outcomes. Children forced into caring responsibility have worse outcomes. Care leavers booted out to cope at sixteen have horrendous outcomes.
We should be encouraging ongoing support for all children, supporting childhood not rushing into adulthood unsupported and acknowledging that strong family relationships are a good thing.