As far as I can tell, the main horror of the story is that he was so DESPERATELY trying to hold on, and then had to experience the terror of it rapidly falling apart and being unable to do ANYTHING about it...
Honestly, like a lot of Lovecraft stories, I don't feel it's so much terrifying as it is surreal, but there is something to be said for the notion of experiencing the effects of actively biodegrading and being unable to do anything about it, all because the stores that sell the mechanical part you need are all closed (or being one of the people trying to help such a person, without fully understanding what's going on).