And for the price of flavor fail that causes a powerful archmage to suddenly forget basic utility spells and caster players SITTING ON THER ASSES NOT DOING ANYTHING IN COMBAT because they had to put all these utility spells into their spell slots. And price of arguments about who takes what utility spells, and frustration of players really excited to try new spell but never being able to prepare it because utility and other necessary spells need to take priority...it kinda exposes vacian casting as a bad rpg system.
I think it's a bad design when you do stuff like that and yet still insist on every adventure to begin with the training wheels. It also still opens the issue of multiclassing and how players use early levels to grab things without comitting to anything. I have already seen this with Paladins, when players will take two levels and then do everything to have lore-wise excuse to avoid third one, because that's comittment to specific oath and players don't want commitment. I once had a player who would rather put off leveling up and then gave Paladin levels away entierly, than pick up an oath. Now this issue will spread to Warlock, when it was once a class that gave you best connection between the PC and the setting. It serves no one but powergamers and murderhobos.
I had so much fun teasing potential patrons for PCs willing to take Warlock levels, even gave freebies like "if you want to take this entity as your patron, you can use your Intelligence instead of Charisma for Warlock levels", which allowed to do this whople "temptation" aspect naturally. In 5.24 version I will have to ban multiclassing into Warlock and never start a game before level 3, because one of best things go removed for the sake of powergaming convenience.
And what patron can do? GFet clowned on. They gave someone power for free, they cannot take it back (by the rules Patron cannot depower the Warlock) and cannot really do anything but seethe. It just takes great powers that were supposed to be major players of this world and makes them idiots the players can play for fools. And if you keep pressing the issue, then you're bad DM ruining player's fun or build.