Indeed it should not be balanced. They should make the effects as fun as possible for the majority of the playerbase.
Close to equally as balanced as possible means in practice having evert effect being pretty much the same. No body would ever want that.
The game doesn't force you to take the most expensive effect. People around you do. Stop listening to them.
The unhealthy obsession with the meta and balance people have pretty much directly lead to what those same people consider to be bad about the game,as in classes being watered down and too streamlined.
You can't have all classes being distinct and unique and also have them do the same damage,and taking risks to create something new is also a nightmare when it comes to balance,but people don't understand that,nor do they understand that they asked for it.
Game devs should still try to balance stuff,but without having to remove the fun from a game
In my eyes the unhealty obsession for optimizing is very common not only in wow but many other games as well. I think major factors are the availability of information and the relevance of streamers etc. Many people (me included) would first read/watch guides before they pick up a new game, as the obsession of doing everything right from the beginning is so oppressive. Every game has some kind of meta, perfect builds and so that people will cater to.
For wow I don't see this trend leaving. We had it for azerite traits, we had it for corruption and it will be the same for covenants in shadowlands. People will sim every ability and everybody wants the best thing even though it is just small percentages ahead of other options. Whats the solution to this? I really don't know as this is as I said a common trend through gaming. Maybe one solution would be, that all covenants abilites should have the same effects but will alter in purely cosmetic aspects. But then again this would be just another step into streamlining the game. Maybe the biggest solution would be a change in the community. People shouldn't force min-max and meta things in every aspect of the game. Sure if you're at the top % of players you have to get every bonus you can, but there really is no need for that if you play this game casually with friends or random people
You're neglecting the universal benefit for GW across all classes. Stat corruption usage massively differs across the class and not everyone was maxing out their corruption budget on a singular stat corruption. GW DPS per corruption point barely changes across specs. Just because some classes had insane benefit from stacking a singular stat does not change the fact that GW was strong as hell.
Bottom line is when you look at damage specific corruptions only, GW was miles ahead of the competition in DPS per corruption point.
It's a problem that cannot really be fixed outside of devs forcing people to have fun,not many devs are willing to actually take that risk,and even less are good enough to make it work well.
Until then,most players will always prefer to be efficient instead of trying to have fun,even if it ends up making them hate the game
The game has still some balancing issues, but Blizzard doesn't care for them except for GW now, after all these "I got several GW procs"-luckers have had a great time, especially in PvP.