This isn't just about SuperMan though, it's about the fact that there are limited plots.
When there are limited plots, there is a very high chance that there will be an FC involved who can just overwhelm the problem and the OC's have nothing to really shine while doing.
In a true open world where there was infinite plots and things going on all the time then yeah OC's might have a place alongside the FC's but...
That isn't fucking reality.
Anyone trying to claim that there are enough plots and scenes going around all the time that the FC's can't, and won't, be involved in is literally sticking their head in the sand and living in a dream world of their own creation.
@Coin got to play an OC with power on a scale of an FC being a Skrull Green Lantern, congratulations, you got to be the special snowflake.
Most OC's don't get that chance or opportunity getting shut down and even when they are approved, they usually cannot meaningfully impact any scene that has FC's running around in it.
I don't know why this is so hard to understand, MU'ing is not a perfect environment. Every story does not get told. Things are not equal.... unless everyone /is/ given the same starting rules, and FC's are /built/ with the same rules as OC's.
But that doesn't seem to happen in my experience.
EDIT: And fuck you @Coin for saying I want my OC's to be as powerful as Superman. Way to try a bait and switch with an insult to go with it.