Why can't there be non-orc worshippers of Gruumsh?
Why must Gruumsh be the only orc god that matters? Even the Drow have more than Lolth, with Elistraea being of note in this example.
You seem to miss that this has already happened. The game hasn't fully caught up to it, but Orcs already have the same moral flexibility as humans, elves, gnomes, Dragonborn, dwarves, halflings, Goliaths, ect ect
It isn't like the Ship of Theseus at all. I know this for a fact, because I've seen orcs in other fantasy worlds that aren't universally evil, and have a culture that can be good and evil, and they are still recognizably orcs.
I guess I'm still just flummoxed why you think this is impossible, or why this is going to inevitably lead to re-education camps, or them just being boring humans. There doesn't seem to be this issue with any other situation, so why is this so hard for orcs?