By "almost the same" you mean a LOT better. Plasma Brand's direct hit, if all the stars align, can crit for something like 2.5-3k, OS can crit for over 4k.
But apart from that, you're right. It's laughable how Watchman dots are so much more effective.
I agree this dot needs to be under 9 seconds, but really you're comparing Vigilance dot damage (which is in all reality not even a full dot damage spec) to Watchman, which is really a pure dps dot damage spec.