I think the problem is the vast amount of work that these musicians produce, if you do a Pink Floyd and do 1 every 3 years or so you can always put out the best
Hmmm... it seems to me that bands are much slower to put out new albums these days. Two or three years is pretty much standard now. Meanwhile, the prog legends were cranking out masterpieces once a year or more. Let's look at four of the most popular 70s prog bands in their prime for five years (1970-1974):
Genesis (5): Trespass, Nursery Crime, Foxtrot, Selling England, The Lamb
Yes (6): Time and a Word, Yes Album, Fragile, Close to the Edge, Tales, Relayer
ELP (5): ELP, Pictures, Tarkus, Trilogy, Brain Salad Surgery
King Crimson (6): Wake of Poseidon, Lizard, Islands, Larks Tongues, Starless, Red
Contrast that with 2000-2004 (popular, active bands in their prime):
Dream Theater (2): Six Degrees, Train of Thought
Spock's Beard (3): V, Snow, Euphoria
Porcupine Tree (2): Lightbulb Sun, In Absentia
Ayreon (3*): Universal Migrator 1 & 2, Human Equation
*could be 2 since Universal Migrator was one project released as two albums instead of their usual double album.