Renko has something going for her, which Iwanagahime doesn't have. It's the fact that she's just a girl. This might seem unintuitive, and of course Renko is not "just a girl", but someone very special - but let me explain.

To marry a goddess like Iwanagahime, you would have to become very godly yourself. In Japan, this means extreme ascetism. I think to prepare yourself as a husband for a goddess you would have to do something like full nine months of increasingly severe ascetism. Nine months because it's the human gestation period, a kind of full rebirth. The things you would have to do would be like four hours of sleep, constantly chanting prayers to her, constant purification with cold water and an ever increasingly strict diet untill at the end you would be eating just pine needles or nothing. It's very easy to post about Iwanagahime on 4chan, but few have what it would take to really become her husband.

You don't need to starve and sleep deprive yourself for months for Renko. That's why Renko has a chance, always has had.