I'm a firm believer in skill and experience. Neither of which Brock Lesnar has very much of in terms of being an MMA fighter. He's a great wrestler and he swings lunch boxes at you, but he won 99% of his fights just because of sheer size... Not skill or experience... Size. To be very honest I'm very surprised his career in the UFC lasted as long as it has. It's very sad to see anyone suffer from an illness, especially one that keeps them from doing their job. But to be honest, I think with or without his illness his career wasn't going to be much longer. There are WAY to many good fighters for him to have stayed on top or even in the title picture for very much longer with how little experience and skill he really had. Hate to sound cliche, but size isn't everything, especially in MMA (Royce Gracie... enough said). Brock also won fights because people feared him, but once people lose that fear you lose your leverage over them, and your cofidence also takes a hit. It's WAY more to this than just an illness, or at least that's how I feel about it.
Edit:
I DO think that Brock Lesnar has ton's of potential... and his illness may keep him from showing his potential... and that I will agree is very sad!