Sion's Passive doesn't fit with his kit

As an experienced Sion player and a huge fan of the champion, I have to say that his passive just doesn't do anything for his kit. It's basically the same as karthus with one major difference: People target Karthus early in the fight because he's the carry. They want to blow him up quickly, so his passive is a big help to still do some damage even if you blow up. However, because Sion is a tank, he's never the focus of the enemy team unless everyone else is dead. In his champion spotlight, Phreak even says that hitting Sion during a team fight is "a waste of time." When everyone else is dead and Sion finally dies, the enemy only need one little CC move like a slow, snare, stun, or knock up and Sion's passive amounts to nothing at all.

I would prefer that Sion's passive be changed to something else. I would prefer a passive that would increase his move speed somehow, but almost anything would be better than the current passive.