I am actually glad theymos didn't use the standard slogan we keep hearing all over the place. Those who know what it means, are already not keeping coins with custodians. Those that don't, either don't care or are thinking it's (hopefully) not going to happen to me. Displaying it all over the place will probably not change things too much.
Well, the popular saying isn't exactly beginner friendly either. We have to remember that we will have new users that aren't familiar with what a key is. Some might argue they should know that, but Bitcoin has now started to leak into the non technical user area, and people will generally learn as they go, rather than researching it before. Since, we're pretty much in the fear of missing out era, and people go in blind just in case they get lucky.
I said it before, but we've got to be as clear, and concise as possible without mentioning too much jargon, even if it's the most basic things as a key.