In ten years of smartphone ownership, I've never killed a phone with water damage. I've actually never lost one to any damage at all. Waterproofing is a gimmick (to me).
So upgrade when/if you drop your old phone in water? That's actually the only time I've upgraded to a newer phone, the two times I ruined my old phone with water. If my next phone is waterproof, it might be 10 years before I upgrade. (Right now I'm on a Galaxy Note 4, which I got after accidentally running my Note 2 through the laundry a few years ago; it's more than adequate for what I use it for.)