Double rare cards are, more or less, the same level of rarity as the Pokemon V, VSTAR, and VMAX cards that you may remember from the Pokemon TCG Sword & Shield sets
Players would rather win or lose the game, and if you want to spend 8 mana on something, then dont spend it on Divine Intervention
With a pylon or pole sign, you have a much better chance at being seen over your competitors
It scales with the X value you choose, so its flexible, and it goes right into any artifact combo deck as a way to grab your win condition on the spot
Manabarbs Casting spells with Manabarbs in play is pure misery
Whats more, it grows from not only your noncreature spells (presumably all instants in a deck like this), but also from your creatures with flash