You missed another point of the opener. Krietor's opener is based on spike damage, whereas the DoT opener guarantees full strength of the second set of DoTs. This means the DoT opener is better for sustaining high damage over longer periods of time, whereas Krietor's spikes and drops off. In the long run, that would result in lower DPS. Also, your estimate still misses a vital point: while it may be -15DPS weaker than it should be as you claim, it is still stronger than Krietor's in the long run.