It isn’t an either/or proposition. There’s multiple ways to both have a viewing and keep a body from getting deep into decomposition, and there’s really no issues with the face automatically becoming grotesque. It really isn’t an every time thing. It isn’t even a majority of the time, when the body is kept cool enough. Pinning isn’t the only option for keeping a mouth closed either.
People have done in home wakes way longer than the funerary industry has existed. There’s plenty of options available.
It’s the origins of it.
When it started being used, it was used derogatorily, so the connotation stuck despite it not being inherently racist to shorten a word.
Tbh, it doesn’t have to be a slur still. But it is still used by racists as a slur because of the origins, so it would take a cohesive effort to reclaim and reframe it.