Two parents sell their baby's soul to a fiend, then leave her on the steps of a temple of Helm. The temple raises the child and trains her to be a paladin, but the fiend has its hooks in her. The fiend wants to corrupt the paladin, and one way to do that is to give her access to warlock powers. The fiend doesn't want to 'win' too quickly, because a high level fallen paladin is better than a low level one.
But the paladin-or Pal/War-will be trying to do good deeds all her life. Probably be consciously trying to defeat the plans of the fiend. Is that a reason for the fiend to take away her warlock powers? No! That would defeat the fiends own plan!
Elric of Melnibone's patron is Arioch, Prince of Swords, Duke of Hell, and all around proper baddy. Elric was the....least evil...of an evil race. Elric hated Arioch, hated serving him, and tried not to do his bidding. Arioch didn't really directly ask much of Elric, apart from asking him to dedicate the slain to him. Elric's warcry was "Blood and souls for my lord Arioch!"
Arioch wanted Elric to destroy the city of Tanelorn. Elric wanted to save Tanelorn, and successfully defended the city from an army of Chaos demons. Did Arioch take Elric's powers away? No. Even at the end of the world when Elric personally slew the gods of Chaos-including Arioch-Elrics powers were never taken away.
The comic character Spawn was given powers by the Devil. Spawn even had a 'Power Clock' which ticked down whenever he used the supernatural power bestowed upon Spawn by the Devil. Spawn knows that when the clock reaches zero then he'll be under the Devil's thumb for eternity, a fate he badly wants to avoid!
But Spawn uses his power to work against Satan! Does Satan take away Spawn's power? No, even to the point where Spawn replaces Satan as the ruler of Hell.
Some of my worst experiences in 40 years of D&D is when DMs take away my agency over my PC. They do this by picking on any PC who, conceptually, gains some or all of their powers, their game mechanics, their special abilities, from an intelligent supernatural source. "If you do that then your god will take your powers away", "If you save those orphans instead of these orphans then you'll lose your paladinhood because you failed to save some orphans. Gotcha!"
The DM takes away my agency through the threat of taking away my PC's game abilities unless I do what the DM wants my PC to do. But I play the game to make those decisions myself, not to watch the DM play my PC as just another one of his NPCs.
So, in those games, the players quickly learn to avoid playing clerics, paladins, warlocks, druids, whatever classes have their powers taken away by DM whim. Nobody says, "Your fighter is making the wrong choices in my opinion, therefore he no longer gets more than one attack per round, loses his fighting style, and cannot use any abilities of his subclass. Any further infractions and you'll be a 1HD commoner. Because I said so".
I an very glad that 5e PCs cannot have their powers taken away, RAW. If the DM does, he is abusing rule zero to do so, and may lose his players as a result.