Is modesty without hijab possible?

My friend doesn't believe me when I tell her that hijab is an essential and important Muslimah item for humility and modesty. We both agree that wearing a hijab but sporting form-fitting clothes is pointless and hypocritical, but she says that you can wear loose, modest clothing and be halal without the hijab. I'm telling her that it is not the same. Many men have said they've been attracted to women simply by hair, so you could still not be halal. I'm trying to argue that attraction is not the only reason for hijab, but it's also meant to reserve your beauty. Why should you show yourself? Why must you show the beautiful Creation of Allah that is you? Don't you find it precious? Don't you hold it close to your heart, cherish it? What is the liberation or reasoning of "being modest and halal without hijab"? I asked her what the point of the hijab was if you could be modest without hijab. She said there isn't really one, it's optional. How do I counter her argument? My points don't get through to her. Allah doesn't state something without a reason! How can she say "There's no point"? If she is in the wrong, I would like someone pointing out the clear reasons of necessity of wearing hijab if one desires modesty. If I am in the wrong, I would like to know, how can one truly show modesty without it? I am not trying to say that hijabis are all righteous and that non-hijabi Muslimahs aren't, but if one wants to be modest in appearance, I can't understand it.