Losing Turnstile to Graspop after Faith No More would be a deep blow, I must say. Maybe they won't be touring in Europe in July anymore, we don't know that yet. Faith No More certainly will, though. Graspop really needs to stop with that double main stage!
Hopefully, this will be compensated for by other guitar bands (also at the top of the bill). But which ones are there rumors about? I can't think of anything beyond The Strokes (if QOTSA isn't an option either).