Adam and Eve (upon them peace) were sent to earth not as a punishment but in order to complete the earth. Hows that?
Well if someone builds a house and nobody lives in it; what is the use of the house? Until some lives in it.
Likewise the earth was made for mankind and this is the place that we have to try to return to the place that we belong paradise. Using the means of the worship of one God/Allah.
Mankind was never going to come out of Adam until they were born on earth.