...and of course we cannot forget the early prog giants like Yes and Genesis.
Personal tastes aside, wasn't prog still a bit of a niche market even in the 70s?
I don't know; Genesis don't have a platinum record until 1978 (And Then There Were Three), and Yes has The Yes Album, Fragile, and Close To The Edge... but Zeppelin has three DIAMOND (ten times platinum) records in a row (IV, Houses Of The Holy, and Physical Graffiti). Floyd has two (Dark Side... and The Wall), with two multi-platinum records in between.
Genesis didn't tour stadiums as far as I know until at least the mid-80's, Yes did stadium dates on occasion (they played Yale Bowl here in CT in '71, with intermittent stadium dates in the mid-70's), but Zeppelin and Floyd were touring stadia - in certain markets, anyway - fairly regularly in the mid- to late-70's. Still with arena dates interspersed, no doubt, but I think Zeppelin and Floyd were on another level.
EDIT: Saw Elton John; didn't think of him. That's not a bad suggestion either.