It feels like you're viewing this as a purely hypothetical question and ignoring reality. No company is 100% good or bad, and it doesn't make any sense to force all possible interpretations into good/bad.
>If not, why did we trust Apple that this wasn't happening already?
I do not trust Apple. I don't really trust any major tech company, because they put profit first, and everything else comes second. I believe that a company as large as apple is already colluding with government(s) to surveil people, because if a money-making organization is offered a government contract that involves handing over already collected data, and it was kept secret by everyone involved, why would they refuse? I know that's very cynical, but I can't see it any other way.
But that's beside the point, which is that what apple is doing is paving the way for normalization of mass government surveillance on devices that we're supposed to own.
>If they were corrupted, why tell us about this at all?
So that we can all get used to it, and not make a big fuss when google announces android will do the same thing. It's much easier to do things without needing to keep them a secret. This is in no way only about apple, they're just breaking the ice so to speak.