We don't need to mitigate like a Paladin. We just need to mitigate well enough and reliably enough that your average party can be willing to consider DRK as a viable and valuable alternative.

Also, the damage difference between tanks very likely isn't nearly as high over time as you make it out to be. DRK may have higher burst damage and do more to large groups, but its poor sustainability leaves it gasping for air in any fight longer than a few minutes, and trash has never mattered, and never will matter. If it did, we wouldn't be calling it trash.