The thing is that Jayce succeeds. He was right.
He was able to create magic. His idea was revolutionary. He was able to harness the arcane and fuse it with scientific principles to create amazing things. Somewhere along the way he reached his life's goal of giving magic to the world, and he still had so much more left to offer. He had the praise and the accolades and the funding and his face on the goddamn mugs.
And it wasn't enough. Not where it mattered most. Not for who it mattered most.
(Do you think Jayce felt sick, cleaning the dirt and blood from his pristine white coat, waiting? Do you think the memories kept it off, despite the chill in the lab air, with him only re-donning his Man of Progress suit when Viktor leaves for good? Do you think upon his return from the anomaly that he cradles his old mug — the one Viktor bought for him as a joke — suddenly and fiercely glad for his grown out hair and beard because he is not that man anymore. He hasn't been for a long time. Maybe he never really was.
Not that it matters. He has a promise to keep.)