Grantaire has always known the ending.
Even at its best, when spirits were overflowing from hearts and into cups, while Enjolras whipped the room into a wave of patriotic feeling, Grantaire knew. When the crowd surged around them and with them, he knew its heart would fade and flicker. He didn't need Enjolras to tell him theirs was the final barricade. He knew. It had always been the only one.
He could have left. The uprising had been months in the talking, weeks in the planning; even the night before he could have slipped away into the night. He would have lost his friends, but with the shadow of death all but atop them what was one night lost? He could have kept their memories alight, taken the burden of recollection on himself, carried the hope of revolution forward into a more amenable future.
He stayed. Not from any patriotic spirit; his country had never done much, been much, to him personally, although sometimes he wished he felt the noble spirit of change that infused those around him.
He stayed for Enjolras. When he spoke with the fervor Grantaire never felt, the feeling poured out into his listeners, and for a moment Grantaire could sense the edges of the grand purpose that infused Enrolras's entire being. He could shut his eyes and feel patriotism trying to grow in the selfish ground of his heart while his friend's conviction bled out over all of them. And when the dawn broke over him and his belief flagged and failed, it withered only to make more room for Enjolras, for his certainty that Grantaire could never share and for the light in his eyes when he spoke of change and for the softness in his face when exhaustion relaxed the tension of the cause and he drowsed in a corner of the cafe.
With such a man as that, all the knowledge in the world couldn't have kept Grantaire from him. Grantaire didn't fight for the cause or for the people or for freedom. He fought for Enjolras, and life was a small price to pay for that.