Oh yes enslaving demons, why that just make them all good now doesn't it?
I didn't say it made them good, although it's probably not the worst thing a demon could go through given that it's been heavily implied that the lowest of low demons are treated extremely harshly in Burning Legion society. (And those happens to be the demons that Warlocks summon into their service.)
There is no need to delve into demonic magicks when Arcanes proven to be the superior of all magical branches.
Please source where Arcane has proven to be the most superior? As far as I'm aware, Fel magic has still shown to be the most devastating magic when used to its potential. Places like Outlands are proof of that.
Why go that's rout?
Because no matter what advantages a Mage, Druid, or Paladin has against demons, they'll still never truly understand the inner workings and subtleties of fel magic like a group who's studied it for centuries have.
Let me ask you, how far do you think the Argent Crusade would have gotten had they not had the support the Ebon Blade in understanding the full might of the Scourge? Because no matter how effective their magic was against the undead, they still relied on the Ebon Blade to understand other important facets of the Scourge, such as their military tactics and inner workings.
The same is true for Fel users, and an important example of that was how a Demon Hunter (Feronas Sindweller, look him up) effectively knew more about the trickery of the satyrs in Felwood than the nearby Druid school ever did. They were getting sabotaged right under their noses and didn't realize it while trying to heal the land. Where do you think they would have gone had that one Demon Hunter not help them find out about the Satyr? And just to let you in on a little secret... he found out about the satyr through the use of fel magic.
Let's go one more, when in the pivotal moment of Azeroth have demonic fel magics prove cto be the key to saving the world? When have fhe world of Azeroth ever been saved by fel magics.Warlocks leading the charge and saving all of creation, that without warlocks Azeroth would have fallen.
There have not been any examples of fel magic saving the world, just yet, but there have been fel users that have assisted other groups in destroying threats to Azeroth. If you read most of the excerpts from the Council of the Black Harvest, it is confirmed as canon that they gained their powers from their experience in fighting most of the big bads of previous expansions. (Illidan, Ragnaros, Cho'gall, Deathwing, etc.)
That said, I really don't think this is necessary to prove a Warlock can be potentially good. A person doesn't have to save the world, but they can commit a good deed every now and then, and Warlocks are certainly not restricted from doing such when they do their roundabout day to day questing.