I don't quite think the message-doesn't-exist example is caused by "dumbing down" too much. To me, the message "it no longer exists" is not the work of someone who, in trying to make layman understand, overdid it.
Instead, it's the typical kind of mistake made by writers who forget about context.
From the programmer's perspective, I speculate, the error _is_ indeed a case of something being non-existent. The code is written to fetch something from the server, the server says it's not there, and well, the application should tell the user about it.
Within the very narrow context of fetching it from the server, "doesn't exist" is correct. However, in the context of the user who's sitting there looking at the message, "doesn't exist" is simply false.
The message needs to be re-contextualized, such that it makes sense. "The message doesn't exist on the server, it exists on your computer at the moment, but just for a short while, so you better copy it".
It doesn't matter whether the user understand what a "server" is. Simply knowing that the message doesn't exist [at some place], is enough to make the whole sentence sensible.
Like if I go to a car mechanic, who just points to the whole car and say "it's gonna be replaced", it would be confusing. But if he says that "xxx is gonna be replaced", that's perfectly comprehensible, even if I don't know what xxx is.