it is called fast d, not exact d

If you really pronounce it as d, it will sound weird. It is the same issue as mutating [p]->[b]. Sound mutating should come out as natual from fast speaking, not by the purpose.

When american speaks fast, they cannot move the mouth/tongue to a proper position in time, so some sounds are not fully pronounced. Since they own the English, we cannot say they pronounce them wrong.

As the learner, we should pronounce them properly if possible. It will aid us to write the words from the sounds of words and train us to have a right mouth/tongue position.