(1) Don't require the presence of any single character. That character becomes a failure point. (This includes a "Chosen One", but it also includes "these five specific characters must all be present at the Final Summoning". The latter is actually even MORE prone to failure.)
(2) Make the "chosen" status something transferable. For example, if it's the imbued energy of a martyred saint, maybe that energy will leap to someone else if its host is killed. Or maybe there were always more hosts than just the PCs and its just a question of which ones step up.