Still, delaying is much better than blocking. About the high cost (6 ) is not that high, especially if upped (4 ). Dim Shields cost 6 and can be played early in the game, same goes with Turtle & Dusk. I don't find 6 a really high cost.
Let's see why delay > blocking. Nymph Decks, Pharaoh-Scarab decks, Firefly-Queen decks, Arctic Octupus decks, Otyugh decks, Mind Flayer decks, decks with growers and other types of decks which rely on active abilities are countered really hard by Turtle (and there are plenty of these decks around). On the other hand, vanilla creature decks are countered better than Dusk but Turtle can also block them. Because of the above, I think Turtle shouldn't block weapons. It would become broken for sure.
Turtle is not a hard counter to those decks. Hard counter means hard counter - dims hard counter monolife rush, firestall hard counters a light/aether stall, Immorush hard counters discake. Turtle is a soft counter to those decks. And yet it's still much, much worse than Dusk because those decks are so rare and uncompetitive in an unrestricted environment, even if we disregard the huge warping of the metagame caused by sofo, that the benefit from countering them is close to nil.
I have never once seen a deck based around Mind Flayers. Sure, you can put one or two in a grabbow, but most of the time either crawlers, chrysaora or lobo is a better fit than flayers. FFQ decks are far too slow to deal with pretty much anything, and require enough quanta to be extremely vulnerable to denial. Otyugh isn't affected by Turtle Shield until after it's eaten something, and what competitive Oty decks are there anyways? Monowater NQ has troubles with turtle shield, and that's a fair assessment, but by the time Turtle is out (2nd-3rd turn if lucky) monowater has already gotten rolling with enough nymphs to continue the swelling of the horde, and that's beside the point that most time decks already counter nymph decks very well (RT, Eternity in rushes and Sundial/off element stalling in stalls) and so the benefit from countering monowater NQ is reduced. SoP squid decks are obviously not highly affected by procrast. Pharoah/SoR decks also exist to an extent, and here I completely agree with you that procrast significantly slows them down and may even lead to a crucial win. But Pharoah/SoR does not make up nearly enough of the metagame to justify Procrast costing 6 and not blocking weapons.
TL;DR: Decks significantly affected by Procrast are too few and far between for it to stay UP.