But they did keep going beyond the impossible. Weren't you watching when, disregarding everything, Simon kept trying to fight the Anti-Spirals? Rossiu turns against him, does he care? No, he keeps overcoming obstacles, with the help of his Gurren Brigadiers.
He performs the "impossible gattai" with Arc-Gurren and punches a Mugann out of time and space.
Later on, Kittan is the one that goes beyond the impossible and destroys the Death Spiral Machine when the probability of success was 0%. Genome himself admits probabilities are useless when it comes to them.
They escape the Alternate Space Labyrinth, a feat said to be impossible for any inteligent lifeform.
Finally, Simon goes beyond all imagination and combines all the Ganmen with his and his friends' Spiral Energy and creates the Galaxy-sized Tengen Toppa Gurren-Lagann.
So I ask thee, do you still think they stopped surpassing the impossible after the timeskip?