Yes, that's a large part of it. And I agree that DC shot the canon for Superman, Batman and their other heroes decades ago. So yes, the damage has already been done.
But!!! Do you believe allowing other publishers would miraculous restore Superman and Batman's previous continuity? Don't you think that the free-for-all that would result would make things exponentially worse? Continuity? That concept would be just a joke to non-DC writers as opposed to the minor inconvenience it's been for DC writers.
Yeah, yeah, I know. I'm an old curmudgeon, i.e. Silver Age DC fan.