You can't argue some bizarre eugenics experiment is the "best" ending. That's so incredibly abhorrent. I can't even put into words my problems with that line of thinking without risking a perma-ban.
And beyond that, the only difference between synthesis and control is the time that will pass before the Reapers come back to kill us all. In control, you've got a few thousand years. With synthesis, it'll be a few million before new purely organic cells emerge from the raw materials they have on every planet. With species as different as the Hanar, Volus, and Elcor, I have to assume that life always finds a way to bring itself into existence. And without fundamentally altering subatomic particles, a purely synthetic entity can still be created. As the Reapers don't fly off into the nearest star after the green wave hits, they will most likely come back once the organic/synthetic divide reasserts itself.
So since both control and synthesis don't solve the problem and destroy isn't the best ending, there's only two other possibilities. A, a 4th ending is forthcoming where Shepard shuts Star-Jar the hell up and beats Harbinger into a pile of oozing scrap metal, or B, Mike is full of crap. Unfortunately, the latter seems far more likely.
The logic at BioWare astounds me. "Artistic integrity" and forcing someone's idea of a homogenous utopia upon the entire galaxy are not how to convince people you're a decent company anymore. Now you'll have to excuse me. I need to clear my mind of the ethical ramifications of even suggesting synthesis is a good idea. Disgusting.