You need a mix of everything. By the same token Glastonbury wouldn’t be Glastonbury if it was filling the high slots with each years latest hype. There’s acts that have history with the festival and have helped out, earned their corn doing secret slots and guest appearances and helping make the place special. Or have gone over very well with the Glastonbury crowd in the past. Those acts are always in with a good shout of getting a bigger slot at Glastonbury than other festivals and IMO rightly so. That history is part of what sets it aside from others.
I’m a bit surprised by the vitriol that sometimes gets directed towards Elbow, Coldplay etc on here (I know, it’s the Internet) but Emily and co are very good at getting that mix right.