If the festival regresses in size then maybe for Amon Amarth though it's still a poor 3rd booking. I remember them being a headliner on a really small festival near where I live.
When you see a lot of festivals though quickly sell out of tier 1 or primarily/early bird, it's not exactly hard to get a ticket. To be honest think they are asking a bit too much with £75/80 to the current rate. I will bear in mind there that however cheap it was I wouldn't pay it.
There is loads of times Download has been more than 70k though, I'd say that's pretty low numbers for the Premier Rock Festival. If they put the likes of Trivium, KSE and LOG as headliners then their numbers will really come down and being a major festival would come into question. They headline Bloodstock which is a 20k capacity festival. If Download got to a point where they are headlining then Bloodstock won't be a festival anymore and Download won't get the headliners they currently do/did maybe I guess. It'll be interesting how quick Green Day sell day tickets out for Download this year and if they do as any other event that size they will fly out very quickly. Green Day shouldn't have an issue selling day tickets out for an event with Weezer and Jimmy Eat World on the same day. I'd probably attend that over the at the time Hella Mega with Weezer and Fall Out Boy as I like Jimmy Eat World more and FOB who I really wasn't into at that time and didn't know Amyl and the Sniffers then. Hella Mega was a tour about 10 years too late for me.