It is long, long overdue.
Glastonbury prides itself on being one of the most relevant music festivals in the world, and yet they've never had the biggest band of an entire musical genre, and one of the biggest bands in the world, even once. This is a band who has headlined basically every festival in the world at some stage, including other countries versions of Glastonbury (ie. festivals which aren't genre-specific).
I get people saying that they don't want Metallica at Glastonbury, or they don't like them. Personally I think they will win the crowd over - their live show is incredibly intense, professional and well constructed, and you can't really describe to anybody what a stadium metal gig is like without being there - the energy at one really is something else, as the volume is so loud yet so focused.
What I don't get is people who say Metallica aren't a big enough band to headline Glastonbury, which is laughable in the extreme. They're one of the top five best selling acts of the last 25 years, have the best selling album of the last 25 years, and are constantly selling out venues in every corner of the globe. It'd be like me saying One Direction aren't enough a big enough band to headline Glastonbury - there are many other reasons you can say they shouldn't, but to say they're not big enough to would make me look like an idiot.