I think their aggression is qualitatively different from that of other aggressive species such as say, the Klingons, and would have represented a huge
problem. The one Jem´hadar who managed to live without the white and who seemed to have himself more under control, and have the ability to be more thoughtful and measured (raising the question as to what degree the white encourages their aggressiveness) was an anomaly, and despite his best efforts and their own stated desire was unable to change his subordinates.
The Jem´hadar captured as an infant showed himself to be both unable
to change, and even Odo could not influence his behaviour.
The problem with Jem´hadar aggression is that it doesn´t just represent one trait among many, it is all they are
. They have been genetically engineered to functon purely as killing machines, and killing is all they want to do. They do not even feel the need to sleep or to eat. They have no interest in any pursuits other than fighting. There are no females so they cannot naturally reproduce. What kind of society could they ever build?
Either you´d have to do some re-engineering on a massive scale, effectively re-writing their entire blueprint, or else they´d have to be eliminated. This would probably be the kindest thing to do. With the Dominion War over, they´re not really any use to themselves or anybody else.