You don't invite two white mage's to a party, so why invite a SCH and a WHM when they share abilities?
First off, you made alot of good points, but i think ppl are forgetting sch wont be so much a rdm clone as a 2 in 1. you can flip a switch basically and go from slightly gimped whm to slightly gimped blm, but at the same time sch is bring its own gifts to the table as well. Sure a rdm can take on both roles but I dont think it can take on either role as well as the main 2 can, sch will be able to do that I think.
I havent got any job past 47 so all my knowledge is of early levels, so im sure im off in regards to late game dynamics.
But to keep on topic, sch may add a bit of competion for blm, but its nothing to worry about, in the end, any job would technically add compettion.