I'm hang-out friends right now with a girl I dated for years. My inner circle is filled with people that used to date as well.
And it's not like the show hasn't already tried to get them together as "hang out' friend. Both Lawrence and Issa were invited to the birthday dinner, because that circle has connections together that won't stop just because Lawrence and Issa broke up. Derrick still hangs out with Lawrence and their group still gets together. A sign of maturation would be former exes being able to hang out. Or at least the attempt to.
Not sure why you would find that as the most unrealistic thing the show has done
Seems like it fits right in with the kind of modern relationship dynamics they like to cover.