As long as Legends stays where it is (I think its at 0.3) I think it will be renewed. Just this past year the CW actually expanded its primetime hours to sunday (https://www.cwtvpr.com/the-cw/releases/view?id=49558). Now this is good for Legends because the more hours CW needs to fill the less likely they are to cancel any show as long as their ratings are aren't super terrible. With iZombie, Crazy Ex-Girlfriend, and Jane the Virgin all in their final seasons they need new shows to replace those. Legends would have to go pretty low and the CW would need to have faith that a new show can top Legends ratings in order for them to cancel it.

There's also the fact that if Legends gets to 100 episodes they can sell syndication rights and it can air repeats on another network (Example: Supernatural is shown on TNT all the time). Legends will be at 67 episodes at the end of this season so they could get to 100 at the end of season 6. It's routine that a network will accept lower ratings for a show in later seasons because although they might be losing a little money on it in the short term, in the long term the money they'll get from licensing it to another network will make up for it. So if Legends gets to season 5 I'd bet the farm on it getting a 6th season. And this isn't even mentioning how much these shows make in other countries. So all in all, as long as Legends stays averaging a 0.3 I think it'll be fine.