This would fix what? Have you ever been part of a group where a tank tunnel visions into DPS mode? Have you done it yourself? Careful with your statements because you're not only dismissing DRK, you're dismissing tanking all together, including PLD and WAR.
There is nothing wrong with the concept of tanks and healers contributing to damage so long as their primary duties are covered. The thing is though, while their primary duties are typically easy to accomplish, they are also vastly important so the true DDs can do their job. Especially in content where players are pushed. The holy trinity isn't going away anytime soon, so I would think long and hard if you can find enjoyment in this game with the way things are set up now.