No, good corruption does not make item level not relevant because the corruption effect scales with the item level (at least the really strong ones like infinite stars).
And not, you can not compare that with 4set bonus at all. Want to know why? Because you usually get decked in set pieces and it is possible to trade them away.
Yes, it's possible to trade corruption away, but who tf is going to give away a 460+ piece of infinite stars r3 in a raid or m+?
Again, I am very sorry but it seems like you did not look into the system very much, because everything you say is based on false assumptions. Like its ok if you do not care about how the system works, but then simply don't pretend that you do.