Simply calling Jehovah's Witnesses "stupid" for believing all this ignores the very real psychological mechanisms that can affect us all. Personally, I see them as a cautionary tale about how I, too, can be misled by my own delusional reasoning, influenced by my motivations.
Confirmation bias, cognitive dissonance, and denialism all play a role in leading them to find merit in the claims of the Watchtower. As for their motivations? Fear of losing a sense of purpose, comfort in hardship, community, ethical guidance, family, identity, and a sense of security. And, last but certainly not least: they fear God's wrath.
So, I wouldn't call them stupid. I grew up in the JWs and was one as an independent adult for 15 years; 15 years! I wasn't stupid—I just had to free myself from all the wrong motivations that kept me upholding baseless arguments.