The objections remain - I think fundamentally changing every single lifeform (not even sentient species!) is a monstrous act beyond anything else possible. If you honestly think that's better than the option of shooting space ghost in the head and then letting the *next* cycle destroy the reapers then I think we're just coming at it from a completely different moral viewpoint.
The Synthesis ending is obviously stupid. But I'm assuming the hypothical that the beam does what the game tells me.
My moral viewpoint is that I want to save lives. There is zero reason to choose Refuse and punt the problem to the next cycle. You'd be causing far more deaths than Destroy does. We can have a discussion about whether Destroy or Synthesis is the better outcome for the galaxy, but choosing Refuse isn't even on the table. It assumes a Kantian morality so extreme as to contradict the possibility of progressing through the Mass Effect series to even reach ME3's ending.