First, the question remains: Why would anybody cast the apocalypse spell (Dumbledore mentioned omnicidal maniac Dark Lords, though).

Second, I still don't think the True Patronus is all that powerful. It cannot be used offensively against anything other than dementors and it does not defend against any magical, technological, natural, or creative-use-of-magical (such as the Mass Accelerating Charm) other than the killing curse, including ones that are just as fatal and which are probably almost as hard to shield against. Moreover, in noncombat uses the True Patronus gives almost nothing that cannot be given by the mundane Patronus or other mundane magical or mundane technological means. (the only thing I have thought of is an unforgable signal of anti-deathism to people able to understand the philosophy of the Patronus or the True Patronus.