I really dont think you have played DRG much tbh, you are comparing apples to oranges. The basic TT combo is only 20% of what a DRG is doing at endgame, the other 80% are keeping up your DOTs and Buffs and keeping your oGCDs on CD. You say MNK has 3 DOTs to manage, so has DRG: Phlebotomize, Fracture and Chaos Thrust. Thats like saying a SMN is easy to play because he only spams ruin once all DOTs are applied. A DRG cant keep up his buffs if the mob is moving, neither all of his DOTs but can execute his basic combo, a MNK cant get max potency on moving mobs but can place all of his DOTs and Buffs because there is no positional requirement on those.

It boils down to both doing the same, only differently. I do think that MNK is a bit harder but thats only because an additional amount of buffs is harder to manage than an additional amount of off GCD skills.