Sorry I didn't explaine my reasoning. You see we see all Reapers being killed in destroy purging the infulence the Reapers have on Shepard's mind, yet in the process Shepard's synthetic parts will be reraticated. Hence all Synthetic life in Shepard ( the parts that brought him/her back will be no more. Hence even your are partly Synthetic.
EDI, and the Geth represent Shepard's Synthetic side, and the Reapers want to Control it. They have his/her Organic mind, but they can't control his Synthetic side. Hence Control, but if Shepard get's more assets Shepard's organic mind starts to fight back the Indoctrination, hence Synthesis.
The only way to get Shepard's Organic, and Synthetic mind is to Synthesis it.
Also this is why Shepard get's the breath scene because do to how you prepared the galaxy, and whatt your past actions were. Shepard can break free because he/she believes they can fight the Reapers, and win. Shepard sees what their future can be, and why he/she can't back down.
Hackett is reminding Shepard that " This war did not come without a price." Shepard is reminded about Mordin Legion, Thane and everyone else that died. Yes we see EDI, but Shepard thinks that she died, as well as Anderson.
You see in the memorial scene everyone is sad, and if you have mid, or low ems they put Shepard's name up on the wall because Shepard is exepting his/her death, yet in high ems Destroy Shepard refuses because he/she is reminded what he/she, and her/his friends fought for.