Yes Harry you were right but only up to a point.
What would have happened to a fledgling IS had Bush not invaded Iraq and disbanded the Iraqi army is only conjuncture but as you see from that link the remnants of Saddam's army helped transform it into an entity that was capable of taking over large lumps of land and being the threat to the world that it is today.
So Bush must take some of the blame for without the input from the Iraqi army it is unlikely it could have grown as it did.
Oh and it would seem that if Baghdadi is dead one of Saddam's officers will be the one to take over.
If Baghdadi is dead, next IS leader likely to be Saddam-era officer | Reuters