There was obviously more to the 70s than just Zeppelin. Those just happen to be 4 of the greatest albums of all time, and they were all produced by one band in the 70s. But of course, Dark Side, Wish You Were Here, and The Wall deserve mentions as well. So do Exile on Main Street, Who's Next, Let It Be, London Calling...I can go on.
Though I don't agree with the argument that production style can be a barrier to entry. Personally, I prefer the raw production style of decades past to the over the top, bloated variety of today.