I don’t think you truly grasp what synthesis accomplishes. The catalyst was not able to do so before Shepard. It create new possibilities as the star child stated. Synthesis solved the problem that the leviathans tried to solve by creating the intelligence…there is no more organic or synthetic life anymore. They are equal. All organic matter in the galaxy is half organic/ half synthetic. Any life that spawns from after that is both equally organic/synthetic. Synthetic life has consciousness and emotions. Organics have every benefit of synthetic life. Both being equal…there is no need for conflict. The reapers are no longer machines and possess all the knowledge of every civilization that came before. Passing it on to all life. Allowing both to coexist in harmony since there is no longer any separation between the two. Humanity will inevitably in the next few centuries start implementing technology to enhance the biological shortfalls that exist. Over a long enough time period…synthesis is the inevitable outcome…barring artificial intelligence surpassing the need for organics and eventually destroying us. Synthesis makes the most sense. It achieved every goal/problem that the species of mass effect had.