Wizards formerly able to cast the Patronus charm, once they realise that Dementors are not about fear but (much more scarily to them) death, are no longer able to avoid thinking about it in the way necessary to make the Patronus charm (v1.0) work. As knowledge of the true nature of Dementors spreads, ability to make Patronuses lapses near-universally. A key tool for keeping Dementors under control is lost. Dementors become much harder to handle, everyone in Azkaban escapes, many wizards are killed (or worse) by Dementors, and the only way to stop it is to get Harry to destroy them all, which is a bit much to ask of a 13-year-old or whatever exactly he is.