There are DPS guides all over the internet that take two minutes to look through and check if your rotation is correct. There are simple guides, and crazy in-depth guides telling you your BiS and every single cool down you can use and when to use it. Having something implemented in game isn't going to help any more than these guides do. If someone doesn't care about their DPS, then they will simply ignore whatever they implement in game.

For the people that do care, they most likely parse themselves, read detailed guides and practice their rotations.

If someone doesn't care about their DPS, nothing will make them care, whether it's an in game feature or not.