So the entirety of English society helped pay to make Rowling incredibly wealthy. That seems like a pretty massive concentration of wealth for little benefit to society as a whole aside from some nice pablum to digest.
Plenty of UK bands from the 60s to the 90s have mentioned unemployment benefits as being an important factor in allowing them to exist before they got famous.
Noone is to my knowledge saying she did not also have a substantial impact on it herself - after all, we're not swimming in people who have achieved the same. But there can very well be more than one contributing factor.
To grasp onto your last statement: Try writing a book when you're an unemployed, uneducated mum if she did not have a working welfare system to "break my fall" as she herself put it.
It was her effort, but the welfare system made it possible for her to put in that effort.
EDIT: Though just for the record, as rich people go she is definitively one of the most sympathetic, not least for the way she has made it exceedingly clear that she believes it is fair for her to pay her taxes in the UK without complaint, given what she believes she owes to the system in the first place - and to the extent that she explicitly hit out at the conservatives for wanting to cut taxes for the rich.