There's really no evidence or reason to claim that DRK is going to have lowest threat in trash pulls. Even if Unleash remains at 50 potency, that has nothing to do with threat. Flash is an enmity generating skill and it doesn't even do damage. So it's quite clear that they can adjust the level of enmity a skill does regardless of whether or not it even does damage.

So while losing 50 potency would reduce the amount of enmity generated, it's perfectly possible that they increased the base amount of enmity the skill generates to compensate.

And all that being said, even if you completely ignore Unleash, DRKs still have Abyssal Drain.

DRKs will still be the easiest tanks for trash pulls and large group pulling.