Why do we still have enmity?

[Reynhart]

Except that your "solution" doesn't really do anything about enmity control, other than make it so that Tanks simply spam out enmity 24/7 because it will provide the largest boost in rDPS.

Not necessarily. Depending on how Floors are calculated or how enmity tools work, you could switch between optimizing Enmity or DPS.
For example, let's imagine that the effect of Provoke is to put you at the beginning of the next floor.

• If you know you won't be able to generate enough enmity to reach that next floor by yourself by the time Provoke is available, you'd better do your DPS rotation, then use Provoke, and start buiding enmity from that next floor.
  
• On the other hand, if you have some skills that would allow you to reach that floor by yourself, you'd keep your enmity rotation, and only use Provoke once you reach the next floor, thus gaining two floors at the same time.

It's still a bit raw, but I'm pretty sure you could polish this kind of mechanics to make enmity more engaging.