Actually most of their biggest hits were written by David Paich, who also sang their biggest hit (Africa).

If you take a closer look, Bobby's only hit contributions were leadvocals on Hold the line and shared vocals (with Luke) on Rosanna.

Most of the the other single hits from the first 4 albums were sung either by Luke or David Paich: Georgy Porgy, 99, I won't hold you back, Afraid of Love, Lovers in the night ...

Only 5 out of 17 single releases featured Bobby's lead vocals... So saying he was THE key factor to their success is somewhat exaggerated, imo.