My sense is that we're not necessarily asking people to recode all of the pages they've already edited, but rather to use these templates going forward. If someone comes across an image which needs (for instance) a template to show that it's sexually-explicit, and the template isn't there (because the image was uploaded before this policy was in place), the person who notices it can always add the template themselves. That's why we have the question "I just saw a disturbing image with no warning! How do I add a warning?" in this faq.
Re: Not such a wide spread issue to start with