I have to disagree with you here. The default setting is already set to "hide known file extensions" which would mean someone would have already had to change that setting to encounter this warning in the first place. That would indicate to me that user already can be excluded from your "typical" pool of less technically literate users.
You are ignoring the fact that people who wish to disable this notification are clearly not in that pool of less technically literate users either. I understand the intention to protect less educated users but why ban the more literate ones from disabling [what becomes] an incessantly persistent warning about something they're already well aware of.
I am a web developer and I have to say "OK" to this prompt about hundred times a day, no joke.
If I wanted to be constantly blocked on how to configure my machine I would buy a Macintosh.
[a]