Besides, this is the game guide's own description of the monk's resource:
"To defeat the inhuman evils they pit themselves against, monks draw upon inner reserves of Spirit, a resource that reflects their personal mastery and training.
Spirit replenishes slowly. It is generated in two ways – by specific skills and attacks, and by the completion of combos. Monks who chain their attacks carefully can continue a near-endless barrage of Spirit-fueled strikes.
Spirit is a valuable resource, but massive expenditures of this force allow a monk to call upon potent dodges and defensive moves, as well as monster-slaying finishing strikes.
Thanks to monks’ physical perfection and martial talent, they are free to deliver as many empowered strikes as their Spirit pools will allow – without waiting for ability ‘cooldowns.’"
The gimmicky part about how 7ss does '777' weapon damage is not worth balancing the skill such that we can't use it when available. Balance it to do the correct amount of weapon damage and cost the correct amount of spirit so that we can use it without a CD... going along with this, Sustained Assault is a pretty flavourless rune effect that doesn't 'feel' right, especially considering this description of how the monk is supposed to work from the game guide.