It really depends on your tonality when you say it. In either case, saying "Are you sure increasing your user retention isn't worth something to you?" is just going to be met with another "No, we don't need it" from the other end.
"So you have perfect user retention then?" is a better question because you know for a fact that they can't have 100% user retention and they know that as well so it forces further dialogue.
"So you have perfect user retention then?" is a better question because you know for a fact that they can't have 100% user retention and they know that as well so it forces further dialogue.