It's not sad that he died, he was 85 years old. He gave a lot to the world and had a long (and judging by the last talk of his I attended, happy) life. Pretty good deal if you ask me.
People morn the death of babies, who have accomplished nothing. Ought we not then morn to an even greater extend those who have accomplished something?
We mourn babies and children more because they did not get a chance to live a full life - a loss of potential in more formal terms.
People who reach old age have fulfilled all the potential they can realistically expect to, and death is less unexpected (i.e. "everybody dies, you're lucky to have made it this far"). While we still mourn them, we can also celebrate their life and achievements.