It all depends on what day and what spot. Had to look up the spotify monthly listeners of Volbeat and they are around Sleep Token/Alice Cooper territory. So if they are on a 'smaller' spot and there is still 1 big name to be pulled out of a hat it would make sense to wait for now.
Now if every other festival has booked either Muse or Linkin Park + Volbeat but GMM decides only Volbeat will be enough, that would be dissapointing.
Also, isn't Volbeat just seriously a 'yesterdays news' band at this point? With almost every headliner i can think of reasons to put them there, be it history, legacy, popularity, exclusivity etc. But Volbeat strikes me as the most boring band you could put on the bill as headliner.