I leveled up all the way to 5, my Charisma on my tiefling warlock hit 18 as soon as I hit level 4. I had the option to take a feat but decided against it for the ability score bonuses instead. And while I agree that it doesn't really factor in the fact most DMs allow a party to try multiple times to persuade a person... I feel like to some extent that's 'okay' - but I wouldn't mind a scaling difficulty for success/fails that allow you to use multiple members of your party to try and get the better option to come through. Something like adding a certain number to the DC to show that this character is clearly frustrated by your attempts or something.
