By stating that discard or mill is unfair follows the same logic as saying that big units with comparatively low provisions are unfair as would be the case with Monsters in Gwent. It's not a particularly convincing statement; every faction has their identities and strengths therein.Where did you get the impression that people think that? Not messing with people's decks and hands leaves enough RNG to not be able to play your deck consistently, but at least in a fair way.
We're talking specific mechanics here and a proposal of ground rules to give the game a clear vision and identity. An anything-goes-for-the-sake-of-diversity mentality does not work well, as other card games have apparently shown. Nobody will be saying to make soccer more diverse by letting players use their hands, or also allow hockey sticks and mountain bikes (for one team!) for even more diversity. Again, some ground rules are needed for vision and identity.
Edit, Add: Going back to Usurper, it is objectively clear that it is impossible to properly balance his current ability, never mind how people feel about him. That should be reason enough to change or remove Usurper.
Now of course you can say that discard or mill is unfun to play against; that's purely subjective so I can't really try and convince you otherwise for that.
Usurper isn't dominant by the way; I have over a 50% win rate against players that use him. The way you play around the archetype is by ensuring your deck isn't predominantly focused on the combo(s) you want to pull off with your leader involved. You need to consider straight up value in your deckbuilding; including cards such as the Crones, the Witcher trio, Unicorn/Chironex are all prime examples on how you avoid getting slapped in the face by prison-like decks.
Anyways I have to go right now. I'll continue the discussion with you guys later