That was an awesome, well thought out response! Thank you for that!
And although I don't know as much about the other singers as you obviously do, I know that Bobby was the original and brought to those songs the a major element and vocal tone that made them special.

In listening to the Richard's, I think they are both great singers but I don't believe they could have consistantly sang as high as Bobby did. THe Richard's can hit high notes but they don't have as high of a tenor range that it takes to nail those Toto songs night after night. I believe that may have been the reason they didn't join... They know their own ranges better than anyone and they probably tried to sing with the records and struggled.