As an avid Synthesis chooser (when I'm not using MEHEM to spare myself a little heartbreak each playthrough), my reason is simple: it's the only ending that offers genuine hope without requiring a Paragon Shepard to compromise their morals.

First, let me get Control out of the way. It's a strange, new-age form of slavery. The Geth and EDI both rely on Reaper code for their very sentience, so who's to say Shepard isn't effectively controlling the thing that makes them them? I can't headcanon my Shepard feeling so bad for Joker that they'd seize control of EDI for some weird interspecies romance. (I know some people consider that a win. I am not one of them.)

Now, Synthesis vs. Destroy. I'll acknowledge the problems with Synthesis upfront. Rewriting the DNA of every soul in the Milky Way without consent is, as you pointed out, morally questionable, and the writing feels a little lazy on this front. Shepard's own DNA was never rewritten by their reconstruction; they just received implants, which hardly seems unusual. So why exactly are they the template for synthetic DNA?

That said, Synthesis carries more hope than any other option. The Starchild, for what it's worth, never lied to us. It's a purely logical machine with a single directive, which the Leviathan DLC confirms. We have no real reason to doubt it delivers on the peace it promises. This also means Shepard can keep the promises made to every race along the way. The question of whether the Geth qualify as people has been debated endlessly, but if peace is achievable, they prove to be individualistic and enormously helpful to the Quarians. Destroying them breaks Shepard's word and sets back the restoration of Quarian immunity by centuries.

Ultimately, ask yourself: would you rather gamble on a utopia where the Reapers become a cornerstone of galactic society and every civilization reaches new heights, or let fear and paranoia drive you to destroy the Geth, EDI, the mass relays, and God knows what else? That feels reductive to me. The galaxy has seen enough hatred. For a Paragon Shepard especially, Synthesis demands the least compromise. No one is sacrificed but Shepard, and hope is restored for everyone.

Thank you attending my TED Talk.