I am not sure I understand why people are upset at the idea of trying to help explain complex topics to people who might not understand every aspect of it. For every person who "falsely believes they understand" a topic might there be people who are more engaged and interested in a topic that had previously been obtuse to them?
I don't think anybody is upset about trying to effectively communicate complex topics. What's problematic is "like I'm five", which is sometimes the right level of complexity, but more often not. Depending on the context is can be mildly insulting to one or both parties, and it also suggests that the burden is entirely on the person giving the explanation, rather than the listener making an effort to understand.
Except as nickoneil points out, most explanations on r/eli5 are not actually for 5 year olds and mostly de-jargonized and accessible. They may lack a lot of the subject's nuisance, but they are not insulting.