Rerolling is the act of making a new account to pull for the characters you want. You keep making accounts over and over again, repeating the quests over and over again, to get Primogems over and over again, to pull banners for the character you want, over and over again.
Banners will definitely return. Venti might get a rerun during the Ludi Harpastum if it becomes an event, like Xiao got his banner during the Lantern Festival, thought this might be some time after Hu Tao. There wouldn’t be any reason to make dozens of characters and give players only one single period of time to ever get them. That would discourage new players from joining the game because they’d feel as if it’s too late to get good characters.
There’s no reason to reroll your account, throwing away all your progress, to maybe get the character you want, and repeating the process as nauseum until you do. If you don’t get the character you want, so what? That’s how Gacha games work and it’s not like you have no chance to ever get the character again. Their banners are sure to return. It’s just a matter of time, so save up as many Primogems as you can to spend them for the returning character.