This is actually an excellent idea. Democrats get to move to a place where their liberal views will have plenty of company, without polluting our voting booths. This is an idea whose time has come.
Of course they are still able to vote as an expat in US national elections via absentee ballet; i.e. the same way you vote when you’re an expat living overseas just for work.
And as far as I know, regardless how long you live overseas you can continue to vote absentee as long as you keep-up your registration in the last place you lived.
So in other words, these people with dual-citizenship can vote in elections for both countries. Think about it....that pretty scary.