How is this fair for DRK?

[Xiaoki]

The shorter cooldown doesn't do much, the time between the tankbusters and moments when you need that mitigation is moreso balanced around the 25s of the other short CDs.
Even if as you said, the cost in MP was to offset the shorter recast, the ability itself isn't all that interresting anymore.

There are other uses for TBN that just tank busters, uses that do come more frequently.