This is my take on one of the endings in mass effect 3. It is spoiler heavy.

Asimov's Foundation and Earth describes a very similar situation, which I will refer to.

To recap, at the end you get to choose to either

1. Destroy the reapers with collatoral damage.
2. Control the reapers by becoming them.
3. Merge all organic and synthetic life by using your own cybernetic body as a template.

I chose Synthesis, but not because I thought it would lead to utopia or something. The Catalyst presents this choice as the ultimate evolution. The pinnacle of life. A future in which organic and synthetic combine their strengths for a brighter future. I believe he is wrong.

It is not that Synthesis won't change anything. People may be smarter or stronger, but that is not what the Catalyst tried to accomplish. The Catalyst tried to remove the distinction between two forms of life, so as to stop war. A distinction that never really mattered in the first place, except in our minds.

All Synthesis did in my eyes was removing the illusion that organics and synthetics had to fight because they are different. If people believed they can live in peace, they often will. Just think of Rannoch and the peace solution that can be accomplished.

This doesn't lead to utopia. People are still people, who will undoubtibly screw things up now and then. There will be disagreement, conflict, war, crises and all that.

Also, in my eyes, Synthesis had already happened. Just like one person in Asimov's book says: "Do you think that, in a way, [option 3] has happened already?" Shepard was partially synthetic himself, but it did not change him as a person. He was faster and stronger, but he was still the same loyal soldier, great leader, and bad dancer. He was also the template, so it would make sense that synthesis would be the same for others. That was what the Catalyst did to everyone: in essense, nothing. However EDI and Joker worked together as a team. As did the geth and the quarians. Both started out rough, but they looked beyond their differences and saw they did not need to fight. That is the true solution, the true synthesis.

So Synthesis, as an option, did essentially nothing. And by changing nothing, people saw that they were already the same. Which does solve the original problem of war between synthetics and organics. It won't change people to make them perfect, or make society utopian, but it does remove an illusion. So yes, I do believe this mostly preserves the galaxy.