If the gaseous molecules didn't have any kinetic energy would the surface of a filled balloon still be firm?
If ideal gas molecules had had no kinetic energy, there would have been no gas pressure. Inside nor outside the balloon. The outer gas would have been laying on the ground, the inner gas on the bottom of the balloon.

In such a case, the only remaining force, aside of gravity, would have been the elastic forces of the balloon, pulling the balloon rubber together to its original shape and volume.

For real gases compressed to volume about equal to tight arrangement of molecules, a sudden repulsive force and therefore pressure would have progressively appeared, due repulsion of electrons.