The question isn't whether having access to hardcore porn is harmful. The question is the relative degree of harm.
When a web service can reliably distinguish between adult and child, it can specifically target content to either. Netflix can provide age-appropriate content to its users. That's great.
Groomers can specifically target members of their desired audience. That's not so great. That's bad. That's really, really bad. That's much worse than kids finding hardcore pornography. And that degree of targeting is only possible with widespread age verification laws.