I don't think their titles should be taken away.
He will always be a Prince. He was born a Prince, so if the Dukedom is taken away he reverts to Prince Harry, then she will become Princess Henry, she wants to be a Princess and Americans would think she was "promoted".
There are pros and cons to the Cambridges having a 4th.
It would push down the Harkle children in the line - Lets assume the future and William is the Monarch, It makes Archie 5th in line and the new baby 6th in line ( after George, Charlotte, Louis and Harry ) - Which means they have to get permission from William to Marry
.
I would love nothing more for give birth to a daughter is the mirror Image of Diana - Just imagine how much that would irritate poor little victim Meg.
Lets look into the Crystal Ball to 25 years from now.
Charles will be 97 ( maybe )
William will be 63 ( Might be King by then )
George will be 32 ( probably the worlds most eligible Bachelor )
and
She will be 65 ( allegedly) and divorced